QUARTZ-SAND ROCK. 861 



Wherever coarse mechanical sediments are contributed to the sea by streams, 

 quartz-sand grains are present. Wherever the sea is at work upon the shore, 

 quartz sand is one of the products. At all places where the bodies of 

 water are of a large enough size to produce waves of considerable power — 

 that is, in large bays and along the border of the open ocean — the process 

 of mechanical sorting takes place, by means of which the quartz-sand grains 

 are separated from other minerals. This follows from the difference in 

 specific gravity, size, and shape between the grains of quartz and those of 

 other minerals. Not only are the quartz-sand grains sorted from feldspar, 

 hornblende, and similar minerals, but the quartz sands are sorted according 

 to size, as a result of which in one place the grains of sand are rather large, 

 perhaps averaging 1 to 5 millimeters in diameter, while in other places they 

 are very small, averaging as low as 0.1 to 0.05 millimeter. Of course there 

 are all gradations between deposits of different coarseness, and also between 

 pure quartzose sand deposits and those in which other minerals are impor- 

 tant or dominant. 



However, the frequent almost perfect sorting of the quartz grains from 

 other material, and the arranging of the quartz grains of nearly uniform size 

 together, is a matter of constant surprise. Great formations of sand rock 

 are produced which have not more than 2 or 3 per cent of other minerals 

 besides quartz, and some which have not so much as 1 per cent of any other 

 mineral. Moreover, in some formations, the ratios of the diameters of but 

 few of the grains is as great as 1 to 2, and the majority of the grains do not 

 vary by as much as a third of a diameter. Both the advanced condition of 

 the sorting and the uniformity of the size of the grains are well illustrated 

 by the St. Peter sandstone of Wisconsin. Specimens of this sandstone 

 show but few particles of any other mineral than quartz. 



Mr. Sydney H. Ball made for me a considerable number of measure- 

 ments of the grains of this sandstone from two different localities. From 

 one locality 25 grains were measured, which gave as the average greatest 

 diameter 0.3388 mm., and as the average least diameter 0.2964 mm. Of 

 these grains 21 have major diameters ranging between 0.20 and 0.50 mm., 

 of which 14 are between 0.25 and 0.45 mm. Of the grains measured, 21 

 have minimum diameters between 0.20 and 0.50 mm., and of these, 14 are 

 between 0.25 and 0.45 mm. The maximum and minimum lengths and 

 breadths for individual grains are 0.54 and 0.16 mm., and 0.50 and 0.14 mm., 



