50 PEOCEEDINaS OF THE GEOLOGICAL SOCIETY. 



fluid-cavities, with or without included cubes of alkaline chlorides, 

 would show that the mineral had been formed under considerable 

 pressure, and might thus point out very clearly the origin of the 

 fragment. 



Mechanical Wearing of Material. 



So far, I have considered the original form of the fragments, 

 irrespective of subsequent mechanical wearing. In the case of 

 fine sand the friction on the bottom must be very slight, since the 

 w^eight of the particles is so small, and even a moderately agitated 

 current of water would raise them from the bottom and carry 

 them along almost without friction. "When, however, the grains 

 are larger, there is necessarily more friction — and still more so in 

 the case of subaerial blown sand, since the pressure on the 

 surface over which it is driven would be fully twice as great. The 

 material thus worn off from separate grains, or from pebbles of 

 quartzite or other hard roclts, would often be in such small par- 

 ticles that even its true mineral nature could scarcely be determined, 

 much less the particular kind of rock from which it had been 

 originally derived ; and it would be difficult, if not impossible, to 

 distinguish by the microscope particles thus produced by the 

 mechanical wearing of stable minerals from those due to the 

 chemical decomposition of those which are unstable. 



Chemical Decomposition of Materials. 



The change of felspar into kaolin is so well known that I need 

 describe only the microscopical characters of the resulting mate- 

 rial. When only partially decomposed, felspar becomes more or 

 less opaque, owing to the formation of minute granules, but 

 retains more or less of the original definite optic orientation. 

 When completely decomposed, it is changed into a mass of minute 

 granules having no more definite crystalline orientation than those 

 in a small grain of hardened mud ; and though these granules may 

 cohere sufficiently to keep together, or may be cemented by other 

 substances, yet in many cases they readily separate more or less 

 completely. I have very carefully studied them, since they play 

 such an important part in many stratified rocks. Their shape is 

 very irregular ; but they are not specially elongated or compressed 

 in any particular direction, so that they cannot be called either 

 irregular prisms or plates. Their diameter varies somewhat, but 

 on an average may be said to be about iq^qqq of an inch. Their 

 doubly refracting power must be somewhere about double that of 

 quartz or unchanged felspar, so that grains g-oVcr ^^ ^^ ""^^^^ i^ 

 diameter usually give with polarized light the pale blue-white of 

 the first order ; but those of 20,000 ^^ ^^^^ i^ diameter are too 

 small to give rise to any visible e:ffiects of depolarization. 



Much of what I have just said will apply equally well to other 

 minerals that can be changed into granules more or less closely 

 corresponding to true kaolin, and also to the fine-grained felspathic 



