48 PKOP. T. G. BONNET ON THE [Feb. 1 896, 



schists of Sark, with many of the larger masses of crystalline rock 

 in the Channel Islands, are pre-Cambrian, so that my estimate of 

 the age of these rocks at the Lizard is not affected by any change of 

 opinion as to their origin. But what is the age of the serpentine ? 

 It is older than the troctolite, the two varieties of gabbro, the 

 granite, and the various ' greenstone ' dykes. The last-named, 

 indeed, may be very much younger than it ; for some of them must 

 have cooled quickly, and occasionally seem to be very fresh. Rather 

 similar rocks, and the granite, elsewhere in Cornwall are certainly, 

 at least in many cases, post-Carboniferous. 1 So possibly may be the 

 ' dyke-rocks ' at the Lizard. I think that the serpentine, the 

 troctolite, and at any rate the coarse gabbro, must have followed 

 one another rather quickly; the finer gabbro, however, which 

 sometimes seems to become rather more compact near a junction, 

 may be separated from the others by a longer interval, for reasons 

 which I presume will be sufficiently obvious. But what is the date 

 of the others ? The picrite at Menheniot, which presents resem- 

 blances to some varieties of the Lizard serpentine, is intrusive in 

 Devonian sedimentaries. This question, however, may be ultimately 

 solved by a careful study of the rocks at the Nare Head between 

 Gerran and Veryan Bays. I am indebted to Mr. Pox for my first 

 sight of this most interesting district, of which, however, owing to 

 unfavourable weather, I could only examine a very small part 

 (at Pennare Wallas). Here, going southward up the slope of a hill, 

 we found, first a slaty rock, then serpentine, then outcrops of gabbro. 

 The serpentine, in which a shallow pit has been opened, is much 

 crushed, so that it is difficult to obtain good specimens. It is, how- 

 ever, a dark bastite-serpentine, 2 very like that in Kildown and 

 Kennack Coves, allowing for the effects of pressure. But on the 

 northern side of this pit we find some of the sedimentary rock, 

 which, though much crushed, is evidently a slaty mudstone with 

 coarser bands, formed of materials which seem to have been derived 

 from crystalline schists. The gabbro is identical with that of the 

 Lizard, when the one constituent has become a kind of saussurite, 

 and the other hornblende. The sedimentary rock is doubtless 

 Palaeozoic, although its exact age cannot be determined. The 

 junction here very probably is a faulted one, but so far as I 

 can learn, the crystalline schists do not appear in this district. 

 Thus it looks as if the serpentine had broken through the Palseozoic 

 sedimentaries. If this, on further examination, prove to be true, 

 then it will be highly probable that the Lizard serpentine also is 

 post- Archaean in age. 



1 Or at any rate later than any Carboniferous rocks in that region. But they 

 are older than the rocks now generally regarded as Permian. 



2 Its specific gravity is 2-579 ; that of a specimen from the above-named 

 locality is 2-647. 



