160 THE MICROSCOPE. Nov 



Passing from sea sand to the sand grains of our sand- 

 stones, we should find very much the same thing ob- 

 tained. Sea sand forms, perhaps, the mean between the 

 extremes in composition of the various sandstone beds. 

 These, 1 find, vary from 



72 per cent to 96 per cent for quartz, 

 16 per cent down to per cent lor felspar, 

 2 per cent down to per cent for garnet, 

 5 per cent or 4 per cent down to per cent for mica. 



Occasionally one or two other unimportant minerals 

 are added, but these are of little interest so far as the 

 purpose in hand is concerned. 



Now, why, first of all, this enormous disproportion be- 

 tween the number of quartz grains and the other constit- 

 uents ? Sandstones, as you all know, are derivative 

 rocks. The substances of which they are composed did 

 not exisi from all time as integral parts of the present 

 sandstones. These rocks have been built up of the frag- 

 ments of older, of pre-existing rocks. To anticipate for 

 a moment some of our results, I find that these sand- 

 stones have been largely derived from the waste of gneiss 

 and granite. Now, both of these contain felspar in quite 

 as large proportion as quartz. Why does this not ap- 

 pear in the result — I mean in the re-formed sandstone ? 

 Because the felspar is more easily broken down, breaks 

 down into finer dust, and is, consequently, washed faster 

 and farther away than the quartz, which, being the hard- 

 est, most resistent of our widely distributed minerals, 

 remains behind in larger, heavier grains, and so the pro- 

 cess of separation is effected. The mica, which is also a 

 constant constituent of the original rocks, being more 

 easily buoyed up in virtue of its flat, scaly condition, is 

 easily floated away, and lience the quartz is left, as one 

 might say, in almost sole possession of the field. 



Now, our object is to trace these grains of quartz back 

 to the rocks wlience they wer-e originally derived. If 

 these grains were composed of perfectly homogeneous 



