862 A TREATISE ON METAMORPHISM. 



respectively. From another locality 11 measurements of sandstone grains 

 gave an average greatest diameter of 0.2009 mm., and an average least 

 diameter of 0.1463 mm. At this locality all but 2 of the major diameters 

 are between 0.15 and 0.37 mm., and all but 2 of the minor diameters are 

 between 0.16 and 0.23 mm. The maximum and minimum lengths and 

 breadths of any individual grain are 0.37 and 0.10 mm., and 0.32 and 0.08 

 mm. The above measurements for each of these rocks show a remarkable 

 approximation to a spherical form for most of the grains, and an almost 

 astonishing uniformity of size for the great majority of the grains, although 

 an occasional grain varies considerably from the average in size. 



Besides sand formations which are deposited below the water, sand 

 formations are built up below the air. The most important of these are 

 dunes, the material of which is derived from the sands sorted by the water 

 along the shore. The wind picks up this sand, carries it inland, and thus 

 builds up along large bodies of water, especially such bodies as the Great 

 Lakes and the ocean, very considerable deposits which are similar in mate- 

 rial and arrangement to the sands deposited by water. Such sand forma- 

 tions are, however, more variable in thickness and more limited in extent 

 than sands deposited under the water. In the interior, especially in desert 

 regions, formations of quartz sands may occasionally be built up independ- 

 ently of water, but, in general, interior wind-deposited sands are not well 

 sorted, and consequently contain as abundant constituents other materials 

 than quartz. Therefore such deposits ordinarily do not belong in the quartz- 

 sand family, and they are considered on pages 877-878. 



Since quartz is second in abundance only to feldspar as a constituent 

 of the original igneous rocks, since it is extensively manufactured from the 

 silicates by the processes of metamorphism, and since, unlike many other 

 minerals, it does not readily dissolve or unite as a solid with other minerals, 

 one would expect that very extensive formations of quartz sand would be 

 produced; and to such expectation the facts correspond. Almost every 

 inland lake is building up a belt of quartz sand of greater or less width along 

 the shore. The width of this belt is, of course, dependent upon the size of 

 the lake and upon the depth of the water. The greater the lake the wider 

 is the sand belt, the coarser sands being near the shore and the finer 

 ones farther from the shore. Gradual deepening of the water also is 



