QUARTZ-SAND ROCK. 863 



favorable to a broad belt, for in that case the undertow is effective farther 

 from shore than where the waters are deep. The rule as to gradual increase 

 in depth applies to the ocean where the shore is swept by the full force of 

 the waves. In bays the building up of quartzose sands is largely dependent 

 upon the amount of material contributed to it. If, for instance, a great 

 river flows into a bay of moderate size, the amount of material of all kinds 

 is so great that the wave action is not sufficiently vigorous to sort the 

 material, and pure quartz-sand deposits are not formed. It appears proba- 

 ble that in the past the most extensive quartz-sand formations have been the 

 deposits of moderately shallow waters in great mediterranean seas, as, for 

 instance, the Cambrian sands of the interior basins of the United States. 

 These sands constituted thick deposits in the Appalachian Mountain system, 

 the southern part of Canada, and throughout the entire northern part of the 

 United States. In the western part of the United States almost equally 

 extensive sand formations have been produced In order that thick sand 

 formations shall be built up below the water,. it is, of course, necessary that 

 subsidence shall take place concurrently with the deposition. In this respect 

 one rock formation does not differ from another. 



If the sand grains be supposed to be of uniform size and spherical, and 

 to be arranged in the most compact system geometrically possible, the 

 space occupied by the sand grains is 74 per cent, and the space between 

 the grains is 26 per cent, of the entire space occupied by the formation. 

 (See Chapter III, p. 125.) As shown by Buckley," in the Dunnville sand 

 rocks, which have been partially indurated, the space is found to be as 

 great as 28 per cent, The explanation of this lies in the fact, discovered 

 by Professor Slichter, that under natural conditions sand grains are never 

 arranged in the most compact manner possible. Ordinarily this fact more 

 than compensates for the varying size of the grains and their imperfect 

 spherical shape (see p. 126); and it may be regarded as certain that the 

 quartz grains of well-sorted pure-sand formations, such as the St. Peter 

 sandstone of Wisconsin (see p. 861), occupy not more than two-thirds 

 to three-fourths of the space. 



"Buckley, E. R., Building and ornamental stones of Wisconsin: Bull. Wisconsin Geol. and Nat 

 Hist. Surv. No. 4, 1898, p. 225. 



