Major-Gen. MacMahon — Nature's Manufacture of Serpentine. 73 



from the magma a lion's share of some of the chemical constituents. 

 It may be a selfish thing to do, but, in this sense, some minerals 

 are habitually selfish. The usual rule is that the more basic minerals 

 separate first, and consequently those last formed are more acid than 

 the minerals of the " first generation." 



Owing to the progressive alteration of the magma brought about 

 from the above cause, a gradual change in the composition of 

 minerals of slow growth may, and often does, take place in the 

 course of their formation ; and this modification is marked by 

 corresponding changes in their physical characters, such as colour 

 in transmitted light, and optical properties. Hence what is known 

 to microscopic-petrologists as zonal structure is commonly observed 

 in the minerals of certain rocks — a structure so marked that it 

 sometimes carries with it a progressive change in the angle of 

 extinction from the centre to the periphery of the crystal, indicating 

 in extreme cases a change in the mineral species. Now where, 

 as explained above, the magma becomes more and more acid as 

 comparatively basic crystals separate out from it, the centre of a 

 mineral of slow growth would be more basic than its peripheral 

 portions ; and the centre would be more susceptible to the attacks 

 of aqueous agents than the periphery, because the more basic a 

 mineral is, the more readily it succumbs, as a rule, to ordinary 

 corrosive agents. 



Yariations from the normal type are even more common in the 

 mineral than they are in the animal world, and a very slight 

 difference, no matter from what cause it may arise, in the compo- 

 sition of one crystal as compared with that of another of the same 

 species, or in one part, as compared with another part, of the same 

 crystal, may suffice to give temporary immunity from the attack of 

 a highly dilute acid. The corrosive agent makes for the weak 

 spot, and exhausts itself on the material there before it attacks the 

 less susceptible material in other places. Hence we often see in 

 the examination of thin slices of rock under the microscope, that 

 whilst one olivine has been more or less completely converted into 

 serpentine, another by its side has been left untouched ; and whilst 

 part of a crystal has been converted into the hydrated mineral, the 

 rest has successfully resisted conversion into serpentine. Had the 

 process not been arrested, the less tractable olivines would ultimately 

 have been conquered. 



In view of the above facts, if we grant that water can find its way 

 through the pores of a mineral, I see no difficulty in understanding 

 why the central portions of some minerals should yield to water 

 charged with carbon dioxide, or other chemical reagents, before the 

 peripheral portions. 



I have not alleged, however, in my paper, that the central portion 

 is always the first part attacked. I wrote : — " This alteration does 

 not always begin at the outside of a mineral, and every microscopic 

 petrologist will be familiar with cases in which it has been set up 

 at the heart, and in the internal tissues — so to speak — of a mineral, 

 and when the channels through which chemical constituents have 



