272 Lt.-Oen. McMahon — Conversion of Chlorite. 



is not quite the same as saying that the secondary dark mica, that 

 owes its genesis to contact action, is chlorite converted in situ into 

 biotite. 



I may say, in passing, that I do not deny the possibility of the 

 conversion of chlorite into biotite ; but if this process really does 

 take place it must be a very complex one ; and it does not follow 

 from the admission that the thing is possible, or even from the 

 further admission that it may conceivably take place occasionally, 

 under special circumstances, that I believe this process to be the 

 one usually adopted by nature. 



A very important remark was made by Mr. Teall at p. 221 of his 

 British Petrography (see also the footnote at p. 642 of his paper on 

 the Whin Sill in the Q.J.G.S. vol. xl.) when speaking of the intro- 

 duction of albite into rocks altered by diabase, that the secondary 

 mineral was " due to the actual impregnation of the surrounding 

 sediment by material derived from the eruptive rocks." ^ 



That dark mica is an exceedingly common product of contact 

 metamorphism, in basic eruptive rocks altered by granite, is known 

 to all petrologists. I have several very interesting examples of this 

 among my Himalayan specimens, and I have described one in the 

 Eecords of the Geological Survejr of India, vol. xix. pp. 71-77. 

 The acceptance of a dark mica (it need not always be biotite), 

 however, as a contact mineral, does not necessitate the acceptance 

 of Dr. Callaway's theory. "We are naturally led to ask. Is not this 

 mica the result of direct impregnation ? Must the genesis of contact- 

 mica be preceded by the production of hydrous chlorite? For that 

 is what Dr. Callaway's theory involves. 



It would not, of course, convince Dr. Callaway to assert that 

 dark contact-mica may often be found in rocks which do not contain 

 chlorite, because he might allege that chlorite was there in the first 

 instance and that the whole of it had been manufactured into biotite. 



Our courts of law, in the difficult task of weighing evidence and 

 of deciding which of two conflicting witnesses is to be believed, are 

 very much guided by considering the probabilities of the case. This 

 principle is also a most valuable one in helping scientific men to 

 decide between rival hypotheses. In the present case we are dealing 

 with the contact action of granite. . We know that granite must 

 have consolidated at a considerable distance below the surface of the 

 earth ; and that, consequently, a long period (measured in geological 

 time) must have intervened between its consolidation and its ex- 

 posure at the surface. The sinking of that portion of the earth's 

 crust, and the accompanying deposition of new strata, must have 

 ceased; and a long period of elevation and erosion must have set in. 

 This supposition is inevitable if we have to account for the removal 

 of some 20,000 or 30,000 feet of superimposed strata, and the exposal 

 of granite at the earth's surface. 



' In 1883 I remarked with reference to a Himalayan rock : " these facts appear 

 to me to indicate that the rock was subjected to two different processes of contact 

 metamorphism ; one process — due to heat ; . . . . whilst the second process was 

 probably the injection of matter from the granite rock, possibly in a gaseous or liquid 

 condition, along lines that followed the original direction of lamination or cleavage." 

 Records, G.S.I, vol. xvi. p. 137. 



