Lt.-Gen. McMahon — Conversion of Chlorite. 271 



and French officers in the Peninsular War during brief seasons of 

 armistice. Indeed, one might ahnost go so far as to say that 

 friendly intercourse with these Koman cohorts has imbued Dr. 

 Callaway with a taste for military tactics, and has inspired him to 

 perform a masterly change of front under cover of the active advance 

 of a line of skirmishers. 



We are now told that the conversion of chlorite into biotite is 

 due to contact action ; indeed. Dr. Callaway " thinks it pi-obable 

 that there is not a scrap of biotite in the crystallines of the Malverns 

 which has been produced except by ' contact action.' " I am glad 

 that this fact has been clearly brought out, as I unfortunately 

 received the impression from Dr. Callaway's paper — an erroneous 

 one as it now appears — that the conversion of chlorite into biotite 

 was considered a case of dynamo-metamorphism. 



I cannot attribute this misappi'ehension altogether to my own 

 stupidity. If Dr. Callaway's language had been more precise, and 

 to the point, I should not, I think, have fallen into this error. 



My criticism was expressly confined to the four corners of Dr. 

 Callaway's paper. In that paper he did not say that he considered 

 the change of chlorite into biotite a contact phenomenon. He did 

 not state that the heat required to convert a hydrous into an 

 anhydrous mineral was due to the proximity of eruptive granite ; 

 on the contrary, he used language that seemed to point directly to 

 the generation of heat by dynamic agencies. We were told that 

 '* the temperature of metamorphisra . . . often rose to the point of 

 fusion in the shear zones " ; and further on he wrote, " where the 

 rock is slightly crushed [the italics are mine], and there are no 

 signs of rock-fusion [the fusion depending apparently on the 

 amount of crushing], there is decomposition of hornblende; but 

 when there is intense crushing and shearing, accompanied b}' a high 

 temperature, reconstruction sets in and biotite is generated." 



If Dr. Callaway wished to convert petrologists to the belief that 

 chlorite is converted into biotite by contact action, I think it was 

 unfortunate that he did not expressly say so, and that he used 

 language that seemed to indicate a belief, on his part, that dynamic 

 action was the cause of the conversion. 



Dr. Callaway goes on to remark : " It would therefore appear 

 that General McMahon admits the conversion of chlorite to biotite 

 by contact action ; and if so, I want to know why he opposes my 

 theory." In other places he says that he " does not understand my 

 position," and asks me to "elucidate" it. I trust, therefore, that 

 I may be pardoned for entering into a discussion on this point. 

 I shall be very brief. 



^ I did not admit the conversion of chlorite into biotite. What I 

 said was : " In cases of contact action one can readily understand 

 how aqueous acid vapours, or liquids, emanating from the molten 

 igneous rock under high pressure penetrated the adjoining rocks, 

 and carried with them in solution some of the constituents of the 

 igneous magma." It is certainly fair to imply from this sentence 

 that I admit the generation of biotite by contact action ; but that 



