Br. C. Callaway — Chlorite and Biotite. 217 



V. — On Chlorite as a Source of Biotite : a Eeplt. 



By Chakles Callaway, D.Sc, M.A., F.G.S. 



I AM much, obliged to General McMahon for criticising my 

 recent paper in the Geological Magazine (Dec. 1893, p. 535) 

 on the " Conversion of Chlorite into Biotite," because I have been 

 puzzled to know the exact nature of the opposition to my views on 

 rock-metamorphisra at Malvern, and I have been haunted by the 

 fear that perhaps after all I might have overlooked something 

 fundamental. Happily my curiosity is now gratified, and my fear 

 is removed, so far as one important branch of the enquiry is 

 concerned. As my critic is so good a chemist, I may conclude 

 that the worst has been said that can be said from the chemical 

 point of view. 



The title of General McMahon's paper, "The Eape of the Chlorites," 

 is a little startling. One is reminded of the author of Nana. I 

 must confess to a little grief that any chemical or mineral pets of 

 mine should be charged with impropriety and lawlessness, and I 

 shall try to show that such aspersions are unfounded. 



In his second paragraph my critic concedes a great part of my 

 theory on the origin of biotite. Eeferring to the authorities quoted 

 by me in support of this theory, he states that two of them deal 

 Mviih "cases of contact-metamorphism." He apparently agrees with 

 these writers, for he goes on to say : " 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 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. 



For the last four or five years I have been insisting that, in the 

 Malvern crystallines, biotite has been produced out of chlorite 

 by "contact action." In 1889 I wrote : ^ "The most obvious fact 

 observable in the field is the production of the biotite. Granite- 

 veins, if of large size, that is, six inches or more in diameter, are 

 often surrounded by a sheath of kersantite two or more inches in 

 thickness. Indeed, so uniform is the effect of the contact that the 

 vicinity of veins may usually be safely inferred from the appearance 

 of biotite in the diorite." I do not think I could have spoken more 

 distinctly than this. 



Now if, as I understand ray critic to concede, the hydrous mineral 

 chlorite may, even in the presence of " aqueous vapours and liquids," 

 be transformed into the anhydrous mineral biotite in cases of contact 

 action, why should it be thought necessary to confine the contact 

 effects to the exact point where the intrusive vein touches the 

 enclosing rock ? Contact effects extend to considerable distances 

 from the intruding mass in Cumberland, in county Dublin, and 

 elsewhere, and why should they not extend to a few yards' distance 



1 Quart. Journ. Geol. Soc. Aug. 1889, p. 487. 



