88 KEY. EDWIX UILL AND PEOF. T. G. BONNEY ON THE 



usual filmy " sericite " has been produced. It contains crystals of 

 felspar, sometimes well developed, sometimes rather rounded in 

 outline or cracked. We find also the usual greenish, black-spotted 

 minerals *. Of the two specimens examined microscopically, one 

 closely resembles a specimen of Sharpley rock, except that quartz 

 grains are practically absent ; the other exhibits a more granular 

 structure, rather suggestive of a fragmental origin, and in it aggre- 

 gated patches of a granular mineral, giving rather rich tints of pink 

 and green with the two nicols, are notuncommon ; these, in some cases 

 at least, appear to replace felspar f. Subangular patches also of a 

 brownish mineral are not rare, which has rather weak depolarizing 

 action, and is speckled and bordered with dots of brown iron oxide. 

 This in places seems to be composed of aggregated folia, resem- 

 bling an altered mica, or more probably a chlorite. Their outline 

 is not sufficiently definite to give any real clue to the mineral which 

 has been replaced. Almost certainly it was a ferro-magnesian sili- 

 cate, possibly biotite, but more probably a member of the pyroxene 

 group. In the microscopic structure of the rock there is nothing 

 incompatible with its having been a tuff, but there is nothing to 

 prove it — no confused association of constituents, no bits of indu- 

 bitable scoria ; in short, no definite structure can be detected. But 

 taking all circumstances into consideration, especially the field 

 evidence, a pyroclastic origin seems the more probable one ; that, in 

 any case, the rock is closely related to the porphyroid of Sharpley 

 we think ma}- be safely assumed. 



We pass on to the brecciated rocks, in which tiie quarries are 

 chiefly opened. Here we seem to find almost insensible gradations 

 from the compact felstone-like green rock to rather coarse breccias, 

 which sometimes are also green (fragments and matrix differing 

 slightly in tint), sometimes, as has been said, contain fragments of 

 a colour more or less buff or pinkish. 



That the structure of these rocks has been to some extent modi- 

 fied by subsequent pressure seems incontestable, but the effects of 

 this do not generally appear to be very conspicuous. The purple- 

 streaked breccia most resembles a crushed rock, but microscopic 

 examination certainly does not negative the hypothesis of a pyro- 

 clastic origin. This is favoured, by both microscopic and field evi- 

 dence, in the case of the outcrops already mentioned to the south of 

 the middle pit, and by some formerly seen east of the upper pit (at 

 present,'we believe, quarried away), while the rocks near the summit 

 of the hill, which now seem to be nearly on the same line of strike, 

 are certainly agglomerates. Microscopic examination of the breccia 

 from the pit itself has not helped us much. That the materials 

 have an igneous origin is beyond doubt ; that the whole, if this be 

 a clastic deposit, is practically from the same source ; that in it we 

 have as yet failed to find bits of indubitable ash, which are common 



* See page 81. 



t I have little dovibt that they belong to the zeolite group, but think it safer 

 not to attempt to name them. To do this would involve a long investigation, 

 vvhicii, for my purpose, would be a waste of time. — T. G-. B. 



