300 J. H. CoUhis — Cornish Serpentines. 



Prof. Bonney has expressed the opinion — and I understand Mr. 

 Somervail to endorse this — that the existence of intrusive serpentine 

 in other parts of the Lizard District would be prima facie ground 

 for believing in a like origin for that of Porthalla ; and this, in the 

 absence of positive evidence to the contrary, I should be disposed to 

 admit. The main object of my paper on the Porthalla rocks (which 

 are entirely different from those of Coverack and Cadgwith) was to 

 afford such positive evidence ; and, speaking as a chemist I must 

 reiterate my belief that it is impossible for any one to explain satis- 

 factorily the origin of the specimens I exhibited to the Geological 

 Society in illustration of my recent paper, and especially the 

 specimens marked C and D, without admitting that serpentine has 

 been formed within the substance of the hornhlendic stratified rock. 

 Not only does this latter entirely enclose portions of chemically pure 

 serpentine, but also there are fragments of intermediate composition 

 of every degree visible, some of which I have isolated and analysed. 

 It seems to me, therefore, we must either admit that these have 

 been formed within the substance of the schist, or that they existed 

 as serpentine previous to its formation, and now represent a kind of 

 breccia. I have no doubt that the first hypothesis is the true one. 



Prof. Bonney and others have appealed to the evidence of the 

 microscope in disproof of my conclusions. I have not, however, 

 arrived at these conclusions without much use of that instrument, 

 and I still assert that the evidence — fairly interpreted —is in my 

 favour. The massive serpentine of Porthalla does not present that 

 reticulated structure which has been regarded as the result of the 

 alteration of olivine rocks. But even if it did, this would be no 

 proof of its igneous origin, inasmuch as the essentially Neptunian 

 origin of many olivines has been quite conclusively shown by Dr. 

 Sterry Hunt and others. 



The transformation of one chemical substance into another by the 

 withdrawal of some of its components and the substitution of others 

 is a process which is continually going on in all rocks which are so 

 situated that natural waters are able to percolate through them. It 

 is well known too that all rocks are — some more, some less — per- 

 meable to water. To chemical geologists such changes are matters 

 of every-day observation ; yet, for expressing an opinion that such 

 a transformation has taken place, in the case of the Cornish serpen- 

 tines, my views have been characterized as something like the 

 "transmutation" of the old alchemists (see discussion on my paper.^) 



Such changes have been fully described and traced in the case of 

 serpentine by Dr. Heddle, Dr. Sterry Hunt, and many other writers 

 of repute. Dr. Sterry Hunt states as the result of his own experi- 

 ments that " whenever the comparatively soluble silicates of alkalies 

 or of lime are brought into contact with solutions like sea- water — 

 holding magnesia sulphate or chloride — double decomposition takes 

 place with the separation of a very insoluble gelatinous silicate of 

 magnesia, and further that precipitated silicate of lime is decomposed 

 by digestion with such magnesian solutions, its lime becoming par- 



1 Abstract of Proceedings Geological Society of Loudon, Q. J. G. S. p. 472, 1884, 



