79. 



Major-Gen. MacMahon — Natures Manufacture of Serpentine. 



What I actually said is as follows : " As the above change 

 [viz. the conversion of two molecules of (Mg Fe) O.i. SiOi into 

 H4 (Mg Fe)3 SioOfl] appears to iuA'olve an increase of volume, I 

 think it probable that the passage of carbonated water through 

 olivine results in the removal of some of the ferro-magnesian silicate 

 [viz. Olivine] (without conversion into serpentine) in the form of 

 soluble silica and carbonates." My argument then was this : Olivine 

 is a silicate of magnesia and iron. Its conversion into serpentine 

 involves hydration and an increase in volume ; it is therefore pro- 

 bable that a portion of the olivine is removed without conversion into 

 serpentine in the form of soluble silica and as carbonates of iron and 

 magnesia. My account therefore (to borrow a book-keeping term) 

 only applied — and I thought that was sufficiently evident — to that 

 portion of the olivine that remained behind and suffered conversion 

 into serpentine. 



Suppose a parent gives a gold sovereign to his son but at the 

 same time takes back from him five shillings in silver ; what would 

 be said of his logic if the parent subsequently called upon his son 

 to render an account of his expenditure, and proceeded on the 

 assumption that the boy had spent twenty shillings of his parent's 

 money on "tuck"? Would not the boy be right in saying, "Why, 

 I gave you back five shillings, and I have only to account for 

 fifteen ! " Yet the parent's position in my illustration is Professor 

 Blake's position with reference to his criticism on my paper. I have 

 humbly tendered my account of the fifteen shillings — straight and 

 square — but Prof. Blake says, in effect, " That is all very fine, but 

 your account of the fifteen shillings must be wrong, because you 

 admit having given us back five shillings out of the sovereign ! " 

 Those are not his words, but that is what his criticism comes to. 



In another part of his abstract Prof. Blake writes as follows : — 



"The entrance of water into permeable rocks is easy to under- 

 stand, but beyond this, it finds its way into the heart of the hardest 

 minerals. The writer introduces Boscovitch's theorem, that molecules 

 within a certain distance have a repellant rather than an attractive 

 force, in order to show that there must be molecular interspaces in 

 minerals. Since the alteration of these minerals begins sometimes 

 in the centre, when the channels through which chemical constituents 

 have been abstracted or introduced are too small to be revealed by 

 the microscope, he thinks they must have come in by these invisible 

 pores [in which case, why do they not alter the outside as they 

 pass by?]." 



The objection raised by Prof. Blake is one easily met. Now-a-days, 

 when we can not only study thin slices of rocks under the micro- 

 scope, but also isolate the minerals of which these rocks are com- 

 posed, and subject them to chemical analysis apart from the mass 

 of the rock, petrologists have come to recognize the fact that a 

 gradual change, more or less pronounced, frequently takes place 

 in the chemical constitution of the uncrystallized magma during 

 the gradual cooling and consolidation of an igneous rock. Some 

 minerals crystallize in advance of others, and in doing so withdraw 



