The Yim Tin Tsai Formation (Table 5.4) is exposed mainly in the vicinity of Tolo Harbour, but isolated outcrops occur in the western New Territories, and on Lantau Island, Lamma Island and Hong Kong Island (Figure 5.7). With the exception of tuffite developed at the base of the succession, the Yim Tin Tsai Formation is composed predominantly of lapilli–ash and lapilli-bearing coarse ash crystal tuff. Occasional, thin (<1 m), interbedded tuffite and volcanic breccia layers are the only indications of significant stratification. Towards the top of the formation, the appearance of interbedded epiclastic units marks a transition into the overlying Shing Mun Formation. At its type locality on Yim Tin Tsai, the formation is 200 m thick and unconformably overlies the Tolo Channel Formation (Early Jurassic) (Figure 5.9). The formation includes occasional blocks, up to 0.2 m, of crystal-rich, aphanitic volcanic rock. The weak alignment of crystals, together with occasional fiamme, indicate welding throughout the formation. In the western New Territories, the formation is slightly thicker (up to 300 m thick) and comprises a uniform grey to black, lapilli–ash crystal tuff characterised by elongate or subround lapilli of coarsely porphyritic lava (Plate 5.8). The lapilli typically have diffuse margins and are surrounded by white reaction rims, probably formed when the deposit was still hot. The crystals are mainly of feldspar with subordinate quartz, hornblende and biotite. U–Pb dating of zircon crystals has yielded an age of 164.5 ± 0.2 Ma for the formation (Davis et al., 1997).
Details
Ma Shi Chau and Yim Tin Tsai. Two thin lithostratigraphic units underlie the Yim Tin Tsai Formation and form the basal part of the Tsuen Wan Volcanic Group. On Ma Shi Chau (841050 834510 Jty-1) the Tolo Channel Formation is unconformably overlain by about 5 m of sedimentary breccia. The magnitude of the break in sedimentation is not known although the dip of the breccia beds is less steep than that of the underlying siltstones and the change is sediment type is marked.
The breccia consists of angular clasts of pink or purplish siltstones, fine-grained sandstones and occasional vein quartz in a matrix of pale pinkish brown sandstone. It is trough-bedded in places and appears to be water deposited; however, elsewhere the layers of breccia are irregular and slumped. The clasts within the breccia are cleaved in some cases; in other instances they contain quartz veins and were probably derived locally from the Tolo Harbour Formation. The breccia is overlain by about 20 m of pale greenish grey tuffite and in turn by more breccia. The lowest few centimetres of the tuffite contain clasts of pink and red sandstones, identical to those in the breccia. The outcrop extends along the northwestern coast of Ma Shi Chau but, except for the most southerly part of the section where the base of the tuffite can be seen, exposure is poor.
Southwestwards, some 35 m of the upper breccia are well exposed on the shore at the eastern end of Yim Tin Tsai (Plate 5.A1); these are succeeded by about 50 m of pale grey, lapilli-ash crystal tuff. This tuff is exposed on the peninsula that projects from the southern shore of Yim Tin Tsai (840380 834250 Jty-2). Here a cross-bedded sandstone forms a 0.5 m thick parting which can be traced for 40 m along the shore, within the tuff. In thin section the tuff from the eastern end of Yim Tin Tsai (HK 35, 840860 834530 Jty-3) is seen to be composed of lithic clasts and crystals in a fine ash matrix. Crystals constitute about 40% of the rock, with quartz and plagioclase (oligoclase) in subequal proportions and more abundant than alkali feldspar (microperthite). Biotite is rare and usually converted to muscovite. The matrix of the rock is composed of fine, recrystallised volcanic dust and lithic clasts showing occasional fiamme-like structures. Lithic clasts are of sandstone, or, more commonly, fine ash tuff.
Tuffs of the Yim Tin Tsai Formation are particularly well exposed on the southwestern shore of Yim Tin Tsai, where they form low cliffs. The lowest rocks crop out on the peninsula described above (840440 834310 Jty-4), overlying the pale grey tuff. The precise contact is unexposed but appears to be lithologically sharp and conformable; the same lithological change can be seen at the eastern end of Yim Tin Tsai (840820 834530 Jty-5). The basal 20 m or so comprise dark grey, ash tuff with a faint, fine, crystal alignment and welding fabric. Thin sections (HK 37) show this rock to be a quartz-poor, biotite-rich, welded vitric tuff with a parataxitic texture of elongate, recrystallised, siliceous shards and fiamme. The contact of these lowest beds with the overlying main body of the Yim Tin Tsai Formation appears to be gradational. It is exposed at the eastern end of the island (840790 834610 Jty-6), where weak welding fabrics dipping N at 30-40o indicate superposition. These overlying rocks are grey to pale grey, lapilli-ash crystal tuffs, which are block-bearing in places. Crystals of feldspar are conspicuous, as are those of mafic minerals, biotite and hornblende. Two varieties of lithic clast are distinctive; small clasts of dark grey, fine-grained sandstones or siltstones, and lapilli and blocks (to 0.2 m) of dark, aphanitic volcanic rock with conspicuous aligned phenocrysts of feldspar (Plate 5.A2). Welding fabrics have been recorded, (e.g. at 840730 834640 Jty-7), but are not well developed. The top of the formation is cut out on Yim Tin Tsai by an intrusion of granodiorite.
Island House (Yuen Chau Tsai) and Tai Po Kau. Tuffs of the Yim Tin Tsai Formation are well exposed along the shore of Tai Po Hoi from Island House (836250 834050 Jty-8) to Tsiu Hang (838600 832800 Jty-9). Around Island House the welding fabric is subhorizontal and, on the south of the island, common small lapilli of sedimentary rocks are also arranged in a subhorizontal fabric.
The basal strata of the Tsuen Wan Volcanic Group, intruded by granodiorite and rhyolite, are exposed in the vicinity of St Christopher's Orphanage (838400 832530 Jty-10). A section seen in the road cutting (838150 832330 Jty-11) at Tsiu Hang showed a basal sedimentary breccia overlying sandstones assigned to the Tai O Formation. The breccia includes clasts of quartzite and phyllite and resembles that at the base of the Group on Ma Shi Chau and Yim Tin Tsai. The breccia grades upwards to tuffite with scattered lithic clasts and this tuffite is overlain in turn by lapilli-ash crystal tuff of the Yim Tin Tsai Formation (Table 5.A1). This mixed sedimentary and volcanic sequence also crops out in the borrow area at Cove Hill (838800 830700 Jty-12), where it is similarly intruded by irregular bodies of granodiorite. The top of the Yim Tin Tsai Formation is seen on the western flank of Cove Hill (837550 829790 Jty-13) where it comprises mafic-rich, ash-lapilli crystal tuffs, overlain sharply by block-bearing, ash tuffs of the Shing Mun Formation. The top of the formation also crops out on the hill above Tai Po Kau (836190 832690 Jty-14) where the mafic-rich, lapilli-ash tuffs are overlain by flat-lying mudstones and siltstones of the Shing Mun Formation. The contact is not exposed.
Lo Wai and Shing Mun Reservoir. Minor outcrops of Yim Tin Tsai Formation flank the northern margin of the granites of Needle Hill, in places overlain by block-bearing tuffs of the Shing Mun Formation (836600 829200 Jty-15), while, further west around Lo Wai (830900 826900 Jty-16), there are more extensive outcrops comprising about 250 m thickness of strata, intruded at their base by granodiorite and overlain (830200 827140 Jty-17) by coarse ash tuffs of the Shing Mun Formation. The formation is best exposed in cut faces at the southern end of Shing Mun Road (831400 826800 Jty-18) and along the Shing Mun Reservoir catchwater (830480 826980 Jty-19). Outcrops of tuff around Shing Mun Reservoir (833000 827600 Jty-20) are difficult to assign to a particular formation because thermal metamorphism by the adjacent granodiorite has destroyed original textures and recrystallised primary minerals. However, it seems likely that the Yim Tin Tsai Formation crops out in this vicinity.
Ting Kau to Yau Kom Tau. The Yim Tin Tsai Formation outcrops on the lower, southern slopes of Shek Lung Kung, where it is in contact with fine grained granite to the west, and is faulted against sandstone and granodiorite to the east. When weathered, as at Ting Kau headland (826040 825450 Jty-21), the rock is pale creamish grey, and its crystallinity gives it a granite like appearance. When fresh, as at Sunny Villa, Castle Peak Road (827090 825930 Jty-22), the rock is grey, with prominent, elongated lapilli up to 150 mm long. North of here (827170 825950 Jty-23) the structure of the lapilli is accentuated in the weathered surface of boulders in debris flow deposits. White feldspar crystals in the matrix are up to 7 mm, with quartz up to 3 mm; there is a characteristic speckling of mafic minerals up to 3 mm across, and rare dark lithic lapilli up to 90 mm.
Western Aqueduct. The Yim Tin Tsai Formation occurs in the water tunnel along a 2 km section north of the portal at Yau Kom Tau (827800 825900 Jty-24), again close to the contact with fine grained granite (2. 3 km from the portal) and along a 0.5 km stretch of the tunnel north of the granodiorite, about 1.5 km south of the portal at Tsing Tam (827400 831300 Jty-25). These exposures indicate a gentle northerly dip, with some disturbance next to the steep sided igneous intrusions. The rock is almost invariably very dark greenish grey or black, lapilli bearing ash crystal tuff. The lapilli can be up to 40 mm across in hand specimen, containing feldspar crystals up to 10 mm. Dark lithic lapilli up to 15 mm also occur, in an epidotized matrix dominated both by quartz and feldspar crystals up to 3 mm and by fine ash; mafic minerals are common up to 2 mm.
Tsuen Wan. The formation outcrops to the north and west of Tsuen Wan and lies beneath the part of the town reclaimed from the bay. It is well exposed on the slope of Pun Shan Tsuen, above Tuen Mun Road (828550 826200 Jty-26), and behind Allway Gardens (828600 826500 Jty-27); eastwards to the river at Tso Kung Tam (829100 826800 Jty-28) the intrusive contact with the granodiorite is well seen, or can be accurately inferred from boreholes. The rock in this area is grey to dark greenish grey, and resembles the melanocratic and crystalline granodiorite. However, porphyritic lava lapilli and blocks up to 150 mm are present; at Pun Shan Tsuen there are also dark aphanitic lapilli up to 20 mm across. The rock is composed of feldspar and hornblende up to 3 mm, and quartz up to 4 mm set in a fine ash matrix.
Tsing Yi. The outcrop of the formation on northern Tsing Yi occurs on either side of a granodionte intrusion. In the west is dark greenish grey to black, porphyritic lava lapilli bearing ash crystal tuff. In weathered exposures the rock is pale cream with prominent mafics, quartz and feldspar, but indistinct lapilli. On eastern Tsing Yi, 0.5 km north of Fung Shue Wo, dark greenish grey to grey, porphyritic lava lapilli-bearing ash crystal tuff was exposed in the extensive cuts for housing development (828400 824600 Jty-29). The lapilli are indistinct, and there is much jointing and veining, obscuring the texture of the rock.
Ma Wan. The tuffs of Ma Wan are similar to, or slightly coarser than, those of Tsing Yi. On northwestern Ma Wan (823920 824050 Jty-30) are thin beds of ash tuff, up to 30 mm thick, in a zone less than 5 m wide. Within the beds there are rapid alterations from fine ash to coarse ash; these strata dip roughly north at 56o.
Tsing Chau Tsai area. The crystal tuffs in this area contain a few large, diffuse-edged porphyritic lapilli, and have a generally coarse ash to lapilli grain size, glassy matrix and well-defined mafic crystals; all features which are typical of the Yim Tin Tsai Formation. The tuffs are also characterized by aphanitic, pebble-like, lithic clasts, up to 30 mm across, which, unlike typical clasts in the overlying Shing Mun Formation, do not have altered margins. On the coast (822650 822700 Jty-31) at Tsing Chau Tsai, a lapilli-bearing, coarse ash tuff is exposed. Locally to the west (822100 822320 Jty-32), the tuffs are crudely stratified, and contain quartz and feldspar crystals (pyroclasts) up to 5 mm across.
Siu Ho Wan - Ta Pang Po. Between Siu Ho Wan and Sham Shui Kok, the Yim Tin Tsai Formation comprises lapilli-ash crystal tuffs. The tuff is poorly exposed, but can be correlated with excellent coastal exposures of the member to the east. It forms narrow, easterly-trending bands of country rock between dykes. Between Sham Shui Kok and Ta Pang Po, the formation is characterized by sharp-edged porphyritic lapilli with a linear alignment. There is local hydrothermal alteration of the tuff east of Ta Pang Po, but for the most part it is fresh and unaltered. North of Ta Pang Po, there are uniform lapilli-ash crystal tuffs with some pebble-like lithics. However, there are also sharp-edged porphyritic lapilli.
West of Yam O Wan. For part of the exposed coastline, between Yam Tsai Wan and Ta Pang Po, the tuffs lack the porphyritic lapilli characteristic, and are ash crystal tuffs, with prominent mafic minerals. To the east and west, flow-fabric and abundant sharp-edged porphyritic lapilli are typical. At Luk Keng Tsuen, the Yim Tin Tsai Formation is characterized with a lineation defined by mafic minerals, and small, angular porphyritic lapilli. Although apparently scarce, these characteristic lapilli persist to the southwest. In Luk Keng Bay the tuff has a flow-lineation which is sub-horizontal, or plunges gently to the southsouthwest. The lithic lapilli are characteristically sharp-edged and porphyritic. On Cheung Sok Tsui, the Yim Tin Tsai Formation consists of a light grey, weathering to light creamish-brown, variably ash to lapilli-ash crystal tuffs. They display a prominent lineation, defined by porphyritic lapilli and laths of biotite, which plunge gently to the southsouthwest.
East of Yam O Wan. At Tai Yam, the tuffs are dominated by porphyritic, aphanitic lapilli, 10 to 30 mm in length. These lapilli define a flow-lineation. The rock is also characterized by its pale, glassy appearance, with well-formed mafic crystals. To the east, the flow-lineation is present, but lapilli are not obvious.
Tai Che Tung. Crystal tuff, interpreted as Yim Tin Tsai Formation, occurs on the northern slopes of the summit of Tai Che Tung, between feldsparphyric rhyolite dykes. It is typically a lapilli-ash crystal-dominated tuff, and often has prominent mafic crystals. Dark lithic lapilli are also sometimes present. Locally, the matrix appears to comprise fine ash-grade crystals.
Cheung Sha. Yim Tin Tsai Formation crystal tuffs, containing both large porphyritic lava lapilli and smaller, dark aphanitic lapilli, are exposed on the coast (814000 810500 Jty-33) near Cheung Sha Lower Village. Some of the dark lapilli have reaction (resorption or alteration) rims, although most have sharp contacts with the matrix. The porphyritic lapilli generally have diffuse margins. The tuffs are cut by granodiorite and feldsparphyric rhyolite dykes. Bluish-grey, porphyritic, lapilli-bearing ash crystal tuff is exposed along the catchwater (813300 811100 Jty-34) above the South Lantau Road. It also contains small, dark grey, aphanitic lapilli.
Exposures offshore (811960 809770 Jty-35) from Cheung Sha Beach include tuff and feldsparphyric rhyolite dykes. The tuff contains many prominent lapilli and has a planar fabric as is typical of the Yim Tin Tsai Formation. A very small offshore exposure (813930 809440 Jty-36), about 1 km south of Cheung Sha Lower Village, comprises lithic lapilli-bearing tuff and probably belongs to the formation.
Lithic lapilli-bearing, ash crystal tuff of the formation is exposed on the catchwater (812800 811100 Jty-37) close to Cheung Sha, above the south Lantau Road. Boreholes drilled downslope of the catchwater intersected sedimentary or tuffaceous rocks which are also interpreted as part of the formation. The tuff is characterized by the abundance and variety of lapilli, including small limestone fragments around 10 mm across. In fresh exposures the tuff is typically greenish-grey to light grey, with aphanitic lithic fragments up to 100 mm across, set in a matrix dominated by small crystals. The outcrop geometry implies that the formation is unconformably overlain by lavas of the Lantau Volcanic Group (undifferentiated), as the formation is progressively cut out to the east. To the west, it is truncated by the Tung Chung Fault (Figure 5.A1).
Greenish-grey to grey, ash crystal tuffs are exposed to the west of the Tung Chung Fault, both on the Tung Chung Road (812550 810640 Jty-38), and nearby on the South Lantau Road (812420 810470 Jty-39). The rock contains variable amounts of lithic lapilli, some up to 50 mm, and is dominantly composed of angular to subangular, dark grey aphanitic rock. The matrix is poorly sorted ash, and contains crystal pyroclasts up to 3 mm across. The stratigraphic relationship with the underlying tuffs is unknown, as the formation is cut by feldsparphyric rhyolite dykes to the south. The formation is overlain unconformably by a eutaxitic member within the Lantau Volcanic Group (undifferentiated), and it pinches out below the eutaxite towards the west, and is not exposed in the Tong Fuk area.
Tong Fuk. Grey lapilli-ash crystal tuffs are exposed east of Tong Fuk (811260 809730 Jty-40). A prominent planar fabric, dipping 80o to the south, is defined by elongate porphyritic lapilli and blocks up to 150 mm. Some lapilli have diffuse margins, and appear to grade into the matrix. Smaller, dark aphanitic lapilli, up to 50 mm across, have clearly-defined margins and in some cases have reaction (resorption or alteration) rims. Weathered surfaces expose quartz crystals (pyroclasts) from 1 to 5 mm across, and aligned mafic crystals between 1 and 3 mm in length. Boulders of grey tuff (810360 809220 Jty-41), exposed offshore in the middle of Tong Fuk Mui Wan, contain porphyritic lapilli typical of the formation, as well as small, dark, aphanitic lapilli with reaction (resorption or alteration) rims.
Around the shoreline of Tong Fuk Mui Wan, there are scattered exposures of mafic-rich, lapilli-ash crystal tuff with elongate porphyritic lapilli. On the southwest side (809920 809230 Jty-42) of the bay these elongate lapilli define a fabric which dips northnortheast at 71o. Feldsparphyric rhyolite dykes cut the tuff on the northeast coast. On the south side (810380 808890 Jty-43) of the bay, the tuffs include a porphyritic lapilli-bearing tuff, with a fabric dipping approximately 80o to the southsoutheast. They are intruded by a trachytic quartz monzonite. Elsewhere, this mafic-bearing, lapilli-ash crystal tuff contains dark lithic clasts, up to 20 mm across, and pale porphyritic lapilli and blocks up to 100 mm long.
Yi O. On the Tsing Lam Kok headland (802100 810800 Jty-44), west of Yi O, exposures of crystal lapilli-rich tuff include porphyritic lava lapilli with diffuse margins, and small aphanitic lapilli with reaction rims. Similar tuffs, with prominent mafic pyroclasts, up to 4 mm, and varying contents of porphyritic lapilli, occur around the coast to the north and west of Yi O. West of Kai Kung Shan (801500 809900 Jty-45), tuffs of the Yim Tin Tsai Formation pass transitionally up into finer grained, ash crystal tuffs of the Shing Mun Formation.
North of Yi O, around Nga Ying Kok (802700 811500 Jty-46), exposures of tuff are interpreted as lying close to the contact with the overlying Shing Mun Formation. These tuffs contain quartz crystals up to 4 mm, relict feldspars up to 5 mm, and common euhedral pseudomorphs after amphibole. The relatively coarse, mafic-rich nature of the tuff, combined with the very rare occurrence of porphyritic lava lapilli, are characteristics of the Yim Tin Tsai Formation.
D'Aguilar Peninsula. Coarse ash to lapilli crystal tuff is exposed in the coastal sections south of the old lighthouse at Cape D'Aguilar (844800 807720 Jty-47), where it weathers to a uniform, light grey rock. No blocks were noted in these sections, but in a nearby road cutting (844660 807660 Jty-48) exposures of dark grey fresh rock reveal the presence of blocks of leucocratic porphyritic lava 0.15 to 0.25 m across, displaying characteristic diffuse margins (Plate 5.A3). The lava blocks consist of white feldspar megacrysts up to 8 mm in length, set in light grey to grey aphanitic groundmass. The tuff matrix contains abundant quartz crystals up to 5 mm across and feldspars to 4 mm across, but the average grain size is between 1 and 2 mm. Fresh, single black biotite flakes are visible in hand specimen. In thin section (HK4223, 844490 807900 Jty-49) biotite is also seen to occur as minute crystal aggregations concentrated in small patches. Quartz is dominant among the larger crystals. Smaller plagioclase crystals are ubiquitous, and some are zoned. Near the Wireless Station (844130 807820 Jty-50) there are exposures of weathered fine ash tuff. The Yim Tin Tsai Formation has been intruded by a quartz monzonite to the north, by granodiorite in the southeastern part of D'Aguilar Peninsula, and by mafic dykes and numerous leucocratic quartzphyric rhyolite dykes.
Stanley Peninsula. From the coastline at Wong Ma Kok (839980 806280 Jty-51), inland to the summit of Stanley Fort (840190 806830 Jty-52), coarse ash to lapilli crystal tuff forms a wedge shaped outcrop surrounded and metamorphosed by granodiorite. In thin section (HK4240, 839980 806280 Jty-51) the matrix is almost entirely recrystallised as a result of the thermal metamorphism. The equigranular quartz matrix surrounds less altered, larger quartz crystals up to 2 mm across; occasional zoning of plagioclase crystals was noted.
Round Island. Coarse ash to lapilli crystal tuffs are continuously exposed in the coastal sections around the eastern half of the island. As with the Cape D'Aguilar exposures, the cliff sections show a remarkable lithological uniformity. In thin section (HK4030, 837380 808640 Jty-53) crystals of broken quartz, alkali feldspar and zoned plagioclase averaging 1 to 2 mm are set in a very fine matrix of roughly equigranular recrystallised quartz grains. Hornblende is also present and the rock is speckled with iron oxides. There is no sign of any welding fabric.
Lamma Island. The Yim Tin Tsai Formation crops out over approximately 2.5 sq km of northern Lamma Island, forming a smooth, rounded topography with few surface exposures apart from scattered corestones and blocks on hill summits. Apart from the main outcrop, which extends from Pak Kok southwards to Luk Chau Village, there are some small isolated inliers less than 100 m across, surrounded by the granitic rocks. The bulk of the outcrop consists of uniform coarse ash tuff containing scattered small blocks and lapilli of porphyritic lava. When fresh, the tuff has a bluish grey colour with light grey or brown quartz crystals prominent against a dark matrix containing abundant biotite. This usually occurs as fresh, shiny black crystals, occasionally up to 3 mm (Plate 5.A4).
A distinctive thin fine ash tuff band, exposed in a temporary cutting near Pak Kok (830300 811100 Jty-54), strikes roughly E-W and dips to the south at about 30o. In thin section (HK5096, 830300 811100 Jty-54) the matrix is fine grained and recrystallized, and muscovite and secondary biotite are abundant. Scattered crystals of quartz and microperthite average 2 mm across. A similar fine ash tuff band was noted near Tai Peng (830420 810100 Jty-55). A strong feature forming band of brecciated, intensely hydrothermally altered rock strikes roughly E W across the hill 500 m south of Pak Kok (830300 810950 Jty-56). The rock is iron rich and contains abundant quartz, both as discrete crystals and as anastomosing veins. The original lithology of the rock cannot now be determined and the brecciated appearance may be a result of the metasomatism. Large fallen blocks of this material are found on the beach at Pak Kok Tsui (830120 811140 Jty-57). Similar hydrothermally altered zones 50 to 70 m wide, striking ENE, were mapped north northeast of Hung Shing Ye (830510 809570 Jty-58 & 830580 809370 Jty-59).
A light brown fine grained sandstone band is exposed at Lo Tik Wan (830970 809270 Jty-60) where it appears to dip towards the south. This sandstone is about 10 m thick but is laterally impersistent. In the vicinity of Lo Tik Wan, numerous minor intrusions of quartz syenite are exposed, and the irregular nature of the contacts are well seen in the coastal sections (831060 809450 Jty-61). Farther south, larger bodies of quartz syenite intrude the tuffs, and in places appear to have infiltrated and modified them. Some 300 m east of Hung Shing Ye (830680 809050 Jty-62), fine grained quartz syenite has percolated through the tuffs and large secondary crystals of alkali feldspar have grown within the coarse ash tuff. A similar modification was noted in excavations at Tai Wan Kau Tsuen (830230 809490 Jty-63) where the coarse ash tuff has been thermally metamorphosed and subsequently infiltrated by syenitic fluids. Although the original texture and character of the tuff remain, the quartz syenite has produced a distinctive overprint of aligned alkali feldspar megacrysts.
South of Hung Shing Ye (830470 808550 Jty-64) a body of highly altered tuff, measuring 25 m across is present within granite; it is probably a large xenolith of the country rock. Along the coastline, to the north of Yung Shue Wan (Figure 5.A2), five small areas of coarse ash tuff have been mapped, all occuring as isolated bodies within the granite mass. The tuff bodies vary from 30 to 80 m across and it is not known whether they are giant xenoliths of country rock within the granite or roof pendants.
