ASSOCIATED EOCKS OF THE LIZARD DISTRICT. 923 



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 rockf. 



The results of my examination of the older gabbro and dark trap 

 dj'kes of Coverack, and of these Lizard serpentines, render me rather 

 suspicious of the common statements about the metamorphism of 

 ordinary pyroxenic and hornblendic rocks (*. e. those also containing 

 a fair proportion of felspar) into serpentine. An olivine consti- 

 tuent, as a rule, changes readily into that mineral; enstatite also 

 alters, though more slowly, as we have seen, and as is shown in 

 Brogger 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. 



(6) 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 



* From what has been said it will be seen that most of this is the variety 

 with a metallic lustre, or true bronzite, though it has often been somewhat 

 altered by the action of water, and is now a hydrous bronzite. I bare never 

 seen true bastite in the Lizard serpentine, but believe I have detected it in the 

 Clicker-Tor rock. 



t In addition to the rocks mentioned above, the change of olivine into ser- 

 pentine is well exhibited by the serpentine of Elba. One also, a serpentine 

 with bastite from Sta. Catarina, is very like the Clicker-Tor rock described by 

 Mr. Allport, Quart. Journ. Geol. Soc. vol. xxxiii. p. 422. 



