586 PROF. T. Ge BONNEY ON SOME ROCK- 
coarse type occurring with one more finely foliated variety; a irag- 
ment of a coarse schist or gneiss, very poor in quartz, consisting 
almost wholly of felspar (decomposed) and altered biotite; also 
little bits of schist and schistose rocks, rolled fragments of decom- 
posed felspar (one showing the remains of polysynthetic twinning), 
and grains of quartz, many exhibiting the agglutinated granules 
with irregular outlines so characteristic of portions of gneisses and 
highly altered schists, together with the fragments of quartzites, 
&e. The grit is evidently mainly composed of detritus from Archean 
rocks. The schistose rock named above has not been included in 
the slide. A brownish-green mineral often occurs between the 
grains, which perhaps in the main is altered detrital material from 
a magnesia-iron mica. 
119 (south of Porth-y-corwg, p. 568).—A lead-coloured grit, 
with some angular fragments, one a compact greyish rock, fuil 
2 inches in diameter. The matrix, under the microscope, contains 
only a few grains certainly derived from the coarse gneisses, but 
has an indubitable fragment of mica-schist, resembling those of the 
Menai type, and several bits of schistose rocks and grits. The 
above-named fragment isan impure limestone; it has a very close 
resemblance to a specimen described for Dr. Callaway in a former 
paper (Quart. Journ. Geol. Soc. vol. xxxvii. p. 236, no. 52). 
120 (Tywyn ridge, p. 578).—A slaty, somewhat gritty rock, of a 
grey-green colour, one side of the fragment changing suddenly to a 
very dark green. The slide includes both these varieties, with a small 
piece of what appears to be a fragment of a rather different rock. 
On examination, we find the paler part to present a general resem- 
blance to 116, but to be rather more streaky in aspect. In it are 
scattered fragments of felspar, hornblende, and of a rather coarse 
felspar-hornblende rock. The fragment contains felspar, appa- 
rently crushed, and showing decomposition-products: between many 
of the apparent granules is a pale-green viridite; there is some 
altered biotite and well-crystallized hornblende. The dark-green 
part mentioned above is almost wholly composed of subangular 
fragments of well-crystallized hornblende, with some grains of 
magnetite; the adjoining part of the first-described matrix is gene- 
rally blackened with opacite. 
121 (bid. p. 578).—From a fragment in a dark rock resembling 
the last, especially in its darker part. This fragment is rather 
coarsely crystalline, mottled pinkish and dull green. The micro- 
scope shows that its component minerals have a streaky arrange- 
ment, and by the aspect suggest that the rock is metamorphic rather 
than igneous. The minerals are felspar—too much replaced by 
secondary products for the species to be identifiable—green horn- 
blende, and altered biotite, with some grains of ilmenite and, perhaps, 
magnetite: I think there is a little apatite. The hornblende in 
this rock, in colour and general appearance, corresponds with that 
occurring in fragments in 120. 
122, 123 (aid. p. 578).— Coarse fragmental rocks, both in matrix 
and fragments much resembling the last. A lengthy description 
