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



been abstracted, or have been introflnced, are too small to be 

 revealed by the microscope." As a matter of fact, the alteration 

 sometimes begins from ttie outside. In cases in which an olivine 

 is perfectly homogeneous in chemical composition, one would expect 

 this to be the case — the attack begins from the outside, and works 

 its way slowly and gradually into the interior through " planes of 

 easy solution," or any other planes of weakness, that may exist. 

 When there are actual cracks, the liquid reagent is not too proud 

 to avail itself of the aid afforded by them in the work of sapping its 

 way into the heart of the fortress. 



As for the question whether water under pressure, and under the 

 conditions that obtain below the surface of the earth, can penetrate 

 into the inner pores of a mineral, I must refer the reader to my 

 paper under discussion, and to the instructive papers by Prof. J. W. 

 Jndd, F.R.S., on schillerization and kindred subjects. " We must 

 never forget," he writes in one of his papers,' " that in the deep- 

 seated rocks .... the whole mass, crystals and base alike, must be 

 permeated by liquids and gases." Cracks we can see under the 

 microscope readily enough, and no petrological microscopist is likely 

 to ignore their existence or the part they play in the circulation of 

 underground waters. I likened them in my paper (p. 431) to the 

 small veins and arteries in the human body ; but we want some- 

 thing in the mineral world to correspond with the minute capillary 

 pores through which blood finds its way between the ultimate cells 

 of which the animal body is built up. We cannot see the atoms, or 

 even the molecules, of which minerals are composed ; but we can 

 infer the existence of molecular interspaces, anci we can assure 

 ourselves of the fact that water, under pressure, actually finds its 

 way into these molecular interspaces by what lawyers would call 

 circumstantial or indirect evidence. For instance, when we find 

 lujdroiis minerals occurring in the interior of the mineral constituents 

 of igneous rocks, and when we have independent evidence that these 

 hydrous minerals are of secondary origin, we must admit that water 

 worked its way into the heart of the altered mineral after its birth, 

 unless we are prepared to show that the parent ci'^ystal originally 

 contained enough water to supply the total quantity cpntained in the 

 parasitical minerals generated in its tissues, as w;ell as all the 

 chemical constituents contained in them. The subjfgct is too large 

 to enter into here. 



Prof. Blake proceeds as follows: — "Minute cracks, however, are 

 also present, as is shown by granite and greenstone absorbing water, 

 and containing air, and the capillary action is increased by pressure 

 and heat. [He seems to think, however, that the same conditions 

 would facilitate the introduction of water into the intra-molecular 

 spaces, since] 'heat signifies an increase in the force of repulsion 

 that keeps apart the atoms [sic] and molecules of which these 

 minerals are composed ' [in which case, heating should make a 

 compound take up more water, but, e.g., 'the hydrate of sodium 

 sulphate is more and more thoroughly converted into the anhydrous 

 salt as the temperature increases.' (Fownes.)] " 

 1 Q.J.G.S. 1889, p. 181. 



