414 Transactions. — Geology. 



whether it has been anorthite or labradorite can only be 

 determined by chemical analysis ; at present it is saussurite. 



The question now arises, Wlaat name are we to give the 

 rock ? There is no special name applied to plagioclase-autho- 

 phyllite rocks — apparently because authophyllite is rare. But 

 as anthophyllite is a rhombic amphibole, it may be grouped 

 with hornblende ; so that perhaps the name corsite might 

 be made to include our rock. The typical corsite — i.e., the 

 orbicular diorite of Corsica — is said by Cotta to be composed of 

 anorthite, blackish-green horn-blende, and some quartz. Later 

 writers have omitted the quartz as undoubtedly of secondary 

 origin, and corsite is now defined as an anorthite-hornblende 

 rock. The hornblende is generally the foliated variety called 

 smaragdite, and is supposed to be a decomposition product of 

 augite ; so that, from this point of view, a corsite would be an 

 altered eucrite or gabbro, and in the latter case could hardly 

 be distinguished from euphotide, as restricted by Professor 

 Bonney. Mr. Teall looks upon corsite as a variety of diorite 

 in which the felspar is anorthite.''"' 



Now, although hornblende is undoubtedly often a secondary 

 product after augite, we cannot suppose that all hornblende has 

 been thus derived ; that all syenites have been augite syenites, 

 and that all diorites have been gabbros or dolerites. Evidently 

 hornblende is often an original constituent of a rock, and there- 

 fore, under certain conditions, we have no reason to suppose 

 that it may not become schillerised as augite does ; smaragdite 

 answering to diallage, and anthophyllite to bronzite or hyper- 

 sthene. This being so, it would seem to be advisable to have 

 a name to represent this particular condition of amphibole 

 rocks, and I would suggest that the name corsite be enlarged 

 to include all rocks essentially composed of plagioclase and a 

 foliated amphibole (such as smaragdite and anthophyllite) ; it 

 would then bear the same relation to diorite that gabbro and 

 norite do to dolerite. With the pyroxene rocks the kind of 

 felspar is not always taken to warrant a separate name, as 

 shown by norite, which is a plagioclase-enstatite rock ; and 

 gabbro is often made to mclude eucrite. Why, therefore, 

 should the amphibole rocks be treated differently to the pyroxene 

 rocks ? In the rock from the Dun Mouutam, there is nothing to 

 indicate that the anthopliyllite is a changed pyroxene, but it is 

 itself altered in places into a green fibrous mineral which may 

 be smaragdite. 



* " British Petrography," p. 73, footnote. 



