OS THE ARGH.EAN ROCKS OF GREAT BRITAIN. 531 



ness of it must be exposed. It is a distinctly bedded, strong, hard rock, 

 only slightly fissile ; sometimes quite massive in fracture, and almost like a 

 diorite, but in other places exhibiting very distinct alternating laminations 

 differing in mineral character. The materials are distinctly crystallised, 

 though they do not generally exhibit a definite crystalline outline. The 

 dominant mineral is hornblende, macroscopically a dark green, almost 

 black, microscopically a rich green, strongly dichroic. With it occur in 

 variable quantities the following minerals : quartz, felspar (generally, so 

 far as is recognisable, a plagioclastic variety), and epidote, with more or 

 less magnetite, pyrite, &c. ; occasionally the felspar occurs in rather irre- 

 gular ' eyes,' giving a subporphyritic character to the rock, but as a rule its 

 texture does not very greatly vary. Occasionally the absence of any 

 distinct structure makes the rock almost indistinguishable from a diorite 

 of moderately fine texture ; but often it is beautifully banded, layers of the 

 quartzose, felspathic, or epidotic constituents alternating with those in 

 which hornblende predominates. The bands may vary in thickness from 

 half an inch or more to mere films. They are parallel with the apparent 

 rather ' slabby ' bedding which is generally characteristic of the series. 

 Further, at a place called Hot Point, is a structure which, although the 

 rock is perfectly crystalline, is so marvellous an imitation of false bedding 

 that it is difficult to believe it due to any other cause than the deposition 

 of the original constituents ; these, and indeed much of the group, may 

 possibly have once been basic tuffs. 



(c) Grannlitic Group. — Into this the one last described passes 

 almost insensibly. It is even more conspicuously bedded than the last, 

 and it is distinguished from it by the prevalence of bands of a lighter 

 colour. The latter consist chiefly of quartz and felspar, with a little 

 hornblende or, less commonly, black mica. Sometimes a specimen resem- 

 bles macroscopically a piece of vein-granite, consisting of little else than 

 quartz and felspar : sometimes it might almost be a fragment from the group 

 above described ; but, as a rule, the hornblende is much less abundant and 

 less definitely crystalline. Thus, macroscopically and microscopically, the 

 group is rather readily distinguishable, and, notwithstanding the rarity 

 and inconspicuousness of garnets, I have ventured to call it the Granulitic 

 group. It seems impossible to explain the frequent and repeated in- 

 terchanges of laminoe and bands of these two principal varieties of rock — 

 distinguished so readily by the eye, as the one is a warm, light reddish 

 grey, the other quite a dark grey — except on the hypothesis that they indi- 

 cate original sedimentation, and I have described and figured a case from 

 Kennack Cove which seems indicative of irregular lamination. 1 Owing 

 to the frequent interruption of igneous rocks, and the numerous faults (in 

 which however the throw is probably not great), it is very difficult to assign 

 a thickness to the metamorphic series of the Lizard. The base of the 

 micaceous group is not seen, the hornblendic group must be of considerable 

 thickness, but I should not allot more than three or four hundred feet to 

 the granulitic group. 



Igneous BocJcs. — In the above metamorphic sedimentary series we find 

 the following rocks, which it may be well to enumerate, though it is no part 

 of my present plan to describe them in detail. Some of these also are 

 now entitled to the term ' metamorphic,' as they have undergone great 

 mineral changes. 



(a) Serpentine : a very handsome rock ; sometimes almost black, 



1 q. J. G. 8., vol. xxxix. pi. 1. 



M M 2 



