168 Prof. J. F. Blake— Reply to Crlticmns. 



to demonstrate that the earth is flat, he will, I think, not find it 

 difficult to support his contention by evidence similar in kind and 

 equal in force to that by which Mr. Howorth has attempted to prove 

 " the very recent and rapid elevation of the Himalayas." 



VII. — Eeplies to variotjs Criticisms. 



By J. F. Blake, M.A., F.G.S. 



n^HE ci'iticisms which appeared in the January and February 

 _! Numbers of the Geological Magazine, on certain writings of 

 mine, require some reply. It is obvious that when an author 

 ventures in any way to question or remark upon the results of 

 others, he must expect their opposition to his statements. I have to 

 ofi:er a general apology to all authors on whose writings I have 

 ventured to make critical remarks in the " Annals of British 

 Geology," that I was unable to send them proofs of the articles 

 before publication, as I hope to do in future, as long as the work 

 is continued. In that way misunderstandings would be avoided. 

 However, I must now proceed to business. 



1. General MacMahon complains that a passage put in inverted 

 commas is not his. It should not have been all put in inverted 

 commas, I admit, yet I cannot see that it involves any misrepre- 

 sentation of his views. The removal of silica spoken of cannot be 

 thought to refer to the hvo molecules of olivine dealt with, as that 

 would spoil the equation, but, as the statement is said to be thus 

 bi'ought into harmony with Roth's, it must refer to the additional 

 half molecule, or the fifth if we start with four. So far from saying 

 Gen. MacMahon's account must be wrong, I tried to point out how it 

 really agreed with Roth's, though it appeared to differ. To use the 

 General's illustration, if the son had received a cheque for one pound, 

 and could only get 16s. cash for it, which he spent, his outing, 

 though the items in his account only amounted to the latter sum, 

 would still cost his father a sovereign. If every four molecules 

 converted into serpentine involve the breaking up of a fifth, then it 

 requires five molecules to produce two of serpentine. I am sorry 

 that Gen. MacMahon has not availed himself of the chance I offered 

 him, of giving us the calculation about the volume involved. 



The question I asked as to why the infiltrating water does not alter 

 the outside of a mineral, on its passage to the centre, is satisfactorily 

 answered, provided, of course, (1) that increase in basicity in a mineral 

 of the same general composition renders alteration easier ; (2) that 

 minerals whose centres decay first show a zonal structure, and, if 

 this be applied to olivine, that that mineral should be zoned ; 

 (3) that corroding agents in contact with slightly varying material 

 do not act on the several parts in proportion only to the ease of 

 doing so, but leave the more difficult untouched till the other is all 

 consumed. Is all this well known to be true ? 



There can be no doubt that, as Gen. MacMahon says, water, or at 

 least its chemical elements, finds its way into the heart of minerals; 

 but does it do so as water ? and does it occupy intra-molecular 



