METHYLOTIC ORIGIN OF OPHITES, 39 



of pre-existent rooks as a whole ; that although the change took 

 place step by step, one ingredient giving way before another, 

 still, ultimately, all participated more or less thoroughly in the 

 change. The molecular or crypto-crystalline transformation 

 had thus as its result a lithological transmutation. To be more 

 precise, where a great bed of diallage rock has been converted 

 into serpentine, the felspar as well as the augite has gone to 

 form the latter. This magnesian metamorphosis of labradorite 

 does not seem to have been sufficiently recognized ; but though 

 the general rule is that the augitic mineral is the first which 

 suffers alteration, there are localities in which the felspar would 

 seem to have been first affected. It is true, that in many cases 

 the felspar may not have been converted into true serpentine, 

 but merely into an impure kaolin, which, disseminated through- 

 out a serpentinous basis, may defy individual recognition, from 

 the similitude of kaolin to serpentine itself. Such an inter- 

 mixture may account for the large quantity of alumina in some 

 serpentinous rocks ; indeed, any serpentine rock which contains 

 much alumina may be held to have originated from a primary 

 rock, of which one or other of the felspars was an ingre- 

 dient"*. 



Some years ago Mr. G. H. Kinahan, District Surveyor of 

 the Geological Survey of Ireland, directed our attention to 

 some interesting points in the geology of South Cannaver 

 Island, in Lough Corrib, which ' ' show the gradual change of 

 hornblendic rocks into serpentine." Since then he has pub- 

 lished a brief notice of the island in the ' Explanation to 

 accompany Sheet 95 of the Map of the Geological Survey of 

 Ireland/ p. 33 (1870). 



Having examined this island, we fully agree with Mr. Kiuahan 

 in the view he has taken of this case. Much of it undoubtedly 

 consists of " metamorphic irruptive rocks ; " and the one with 

 which we are more particularly concerned was undoubtedly 

 a hornblendic mineral en masse before it assumed its present 

 character. 



Plate VII. represents a specimen, now a dark olive-green 

 serpentine, having a well-developed crystalline structure. 

 Plate VIII. represents a specimen of grey tremolite, a variety 



* Tj-ans. Koyal Society of Edinburgh, 1878, vol. xxviii. pp. 491, 492. 



