320 Br. C. Callaicay — Reply to General McMahon. 



as I have attempted to show, is being squeezed out from beneath the 

 neighbouring area of depression. Thus, such material will be 

 more and more used up in raising the bordering one of sedimenta- 

 tion, with the ultimate production of a parallel elevation. 



In so far as contraction actually produces splitting, this will, 

 over a region with no especial lines of weakness, produce a very 

 complicated system of joints throughout the entire mass ; and, by 

 thus slowly relieving pressure, bring about the formation of holo- 

 crystalline igneous rocks beneath. 



VIII. — How Chlorite is Converted into Biotite. 

 By C. Callaway, D.Sc, F.G.S. 



GENEEAL McM AEON'S paper in the Geological Magazine 

 for June attacks my conclusion that the Malvern biotite is 

 formed from chlorite, but leaves my evidence untouched. Indeed, 

 a large part of the article is occupied in discussing a theory which 

 I do not hold. I have never said that the chlorite was converted 

 into biotite by contact-action only. That there may be no further 

 mistake, let me repeat that, in the Malvern rocks, chlorite is changed 

 to biotite by contact-action jylus dynamic deformation. 



General McMahon, in rejecting my theory, has rightly felt that 

 he was bound to offer an alternative explanation. He is aware that 

 the materials for the manufacture of the mica must come from some- 

 where, and he suggests that it may have been produced by " direct 

 impregnation " from the granite. That the granite has impregnated 

 the encasing diorite with one at least of its constituents, viz. 

 jDotash, is one of my own points; but that it has bodily produced the 

 biotite is simply impossible. This granite, it must be remembei'ed, 

 consists almost exclusively of quartz and potash felspar. An analysis 

 of it by Mr. Player gives 1-1 per cent, of iron-oxides, and 0-3 per 

 cent, of magnesia. Three analyses of the same rock by Mr. Timins 

 yield about the same results, but one of them contains no magnesia. 

 You cannot get a black mica without iron or magnesia, and it is 

 certain that the granite could not have supplied enough of these 

 bases for its manufacture. The encasing diorite, on the other hand, 

 contains plenty of both bases. 



General McMahon objects to my statement that the conversion of 

 chlorite into biotite is an " observed fact." He writes that " the fact 

 actually observed is the existence of chlorite and biotite in the same 

 rock." So my critic believes that, merely because the two minerals 

 lie side by side, I have inferred that the one was formed out of the 

 other ! I might as well have argued that the sugar in my tea-cup 

 was evolved out of tea. 



I will give General McMahon one or two facts, and allow him to 

 judge whether they support his suggestion. Near the Wych, there is 

 a very clear case of contact between diorite and granite. Both are 

 sheared, and more or less altered. Near the contact, the hornblende 

 of the diorite is decomposed ; chlorite and iron-oxide being most 

 conspicuous amongst the resulting products. Slides of the rock taken 

 at the exact contact, show the decomposed diorite side by side 



