218 Dr. C. Callaway — Chlorite and Biotite. 



at Malvern? In the shear- zones, where most of the biotite is 

 formed, there is usually a plexus of granite-veins, and the diorite is 

 so minutely sheared as to become in a high degi'ee pervious to fluid 

 injections. Where, then, is General McMahon's difficulty ? So far 

 as I can see, he has created it for himself. Let me elucidate thia 

 point. 



After describing the nature of contact action, my critic goes on to 

 assert that "in the case supposed by Dr. Callaway the conditions 

 are altogether different." Are they ? Whence has General McMahon 

 derived his information on the point, if not from my papers ? But 

 I have never described the conditions as " altogether different." On. 

 the contrary, 1 think it probable that there is not a scrap of biotite 

 in the crystallines of the Malverns which has been produced except 

 by "contact action." I have, of course, dwelt upon the important 

 effects of dynamic agencies; but I have insisted with equal emphasis 

 uj)on the changes brought about by the intrusive gi-anite-veins. 



I may observe in passing that General McMahon attributes to me 

 an assertion that in rock metamorphism the combined water of the 

 chlorite has been driven off by heat. This is not quite accurate. 

 In my last paper, I quoted the fact that in the laboratory chlorite 

 loses its water at a red heat as a " proof that the affinity between 

 the water and the other constituents of the mineral is not very 

 strong " ; which is quite another matter. I have not presumed to 

 offer a complete account of the operation of the forces that were 

 working deep down in the crust in Archaean times. Heat, chemical 

 energy, and pressure no doubt co-operated, and who knows what 

 other causes may have assisted in the schist-making ? 



General McMahon's amusing metaphor of the " Kape of the 

 Chlorites " is suggested by a theory of the process of metamorphism 

 which has no basis in my writings. He supposes that his maidens, 

 the chlorites, are in one part of the rock regarded as highly heated ; 

 and his Eoman soldiers, the potash and iron,' in another part, con- 

 sidered to be much cooler. This could by no possibility have been 

 the case. The soldiers and the maidens were living in the same 

 house, only waiting till the affinity between them should grow 

 strong enough to draw them into the bonds of lawful wedlock. 

 My critic's misapprehension seems to have arisen from a confusion 

 as to the order of events in the schist-making process. 



I have frequently described sections which show a passage between 

 a diorite and a biotite-gneiss. Where the diorite is slightly crushed, 

 there is a good deal of liberated iron -oxide. Further along the 

 section, where biotite begins to appear, the visible iron decreases 

 in quantity, having been partially used up, as I have elsewhere 

 shown, in the genesis of the biotite. General McMahon says that 

 this iron (together with potash) has got to rush from one locality 

 to another, like Roman soldiers sacking a town. But this is pure 

 imagination. Surely it is quite obvious that the succession of 

 events which we now witness in geographical series was once a 



1 I have not taten any account of the alumina, since the percentages of that 

 oxide in the chlorite and the biotite are roughly equal. 



