Page:Quarterly Journal of the Geological Society of London, vol. 33.djvu/1053

Rh however, take account of the iron, which must have entered into the composition of the original olivine, replacing some of the magnesia; as this seems to remain, all that would be required would be the decomposition of the original mineral, and hydration of some of its constituents. Mr. Macpherson's paper, which I did not see till I had nearly completed my own, quite confirms my observations; and some of his figures might have been taken from my Cornish slides, so great is the resemblance. As, then, the Lizard serpentine also contains enstatite, an augitic mineral, and even picotite, all constituents of typical lherzolite, we are, I think, justified in regarding it as the result of an alteration (by the action of water, not necessarily at a high temperature) of a great mass of that rock.

The results of my examination of the older gabbro and dark trap dykes of Coverack, and of these Lizard serpentines, render me rather suspicious of the common statements about the metamorphism of ordinary pyroxenic and hornblendic rocks (i.e. those also containing a fair proportion of felspar) into serpentine. An olivine constituent, as a rule, changes readily into that mineral; enstatite also alters, though more slowly, as we have seen, and as is shown in Brögger and Vom Hath's description of the great enstatite crystals from Norway (Monatsber. d. k. Akad. der Wissensch. Berlin, 1876, p. 549). The augite yields also, though, I think, generally subsequently to the enstatite; but if it is an aluminous variety, I believe the silicate of alumina remains to form the rather shapeless dirty-looking microliths which I have often noticed under these circumstances. To remove a felspathic constituent of a rock or convert it into a magnesian pseudomorph would, I think, not be a common operation in nature.

In conclusion we may ask the question, Is there any clue to the age of the various igneous rocks noticed above? From what has been stated, we are, I think, justified in concluding:—

(a) That the sedimentary rock had been metamorphosed before the intrusion of the lherzolite.

(b) That the lherzolite had become serpentine before the intrusion of the gabbro and of the granite.

(c) That the dykes of dark trap are the latest rocks in this part of the peninsula.

I am not aware that there is any proof whether the granite or gabbro is the older rock. Sir H. De la Beche (Report, pp. 99, 173) speaks of the granite as cutting the gabbro; but, as the only evidence seems to be the supposed vein at the Balk, which I am convinced is