652 W. H. SHERZER RECOGNITION OF TYPES OF SAND GRAINS 



Sylvania area embedded in the sandrock, carrying marine fossils, and 

 may be explained as formed during a temporary encroachment of tli(3 

 sea, the granules being distributed through it by wave action or by winds 

 from the neighboring lands. There is a general absence of cementing 

 material in the main body of the Sylvania, except what has been second- 

 arily introduced (figure 3, plate 47), and the rock is characteristically 

 incoherent, crumbling readily in the fingers. At Eockwood, Michigan, 

 the rock is disintegrated by water from a hose, and while in suspension 

 pumped from the quarry. The assorting of the grains is marked, both 

 horizontally over the area and vertically in the bed, and, excepting near 

 the base at a few points, have any granules been observed that could not 

 have been readily handled by the winds. Such gravelly material was 

 reported in the Milan well,^*^ and in Sandusky County, Ohio, by Winchell, 

 where the bed has thinned to but a foot and contains pebbles three- 

 quarters of an inch in diameter.^' This coarser basal deposit is to be 

 expected, and may be interpreted as the beach deposit formed at the 

 stage when the land was emerging from the sea and due to regressive 

 overlap. ^^ Except for the bed of intercalated dolomite just mentioned, 

 and a few inches of horizontal strata at the top, fossils are entirely absent 

 from the formation. That the sandrock was capable of retaining casts 

 of fossils is shown by the fact that marine mollusca are abundantly indi- 

 cated in the uppermost strata, combined with considerable carbonaceous 

 material, presumably due to plant growth. This, again, we should ex- 

 pect, since at the time of final submergence by the sea the sand would be 

 rearranged by the waves and an opportunity offered for the introduction 

 of animal and plant life. 



One further point remains to be noted. In discussing "Desert condi- 

 tions in Britain," in 1896, Goodchild considers all areas of "inland 

 drainage'^ as representing desert conditions,^^ and, based on figures of 

 Murray,^*^ estimates that such areas today, when compared with those of 

 oceanic drainage, are about as one is to fourteen. From this we are led 

 to inquire whether we should not expect to find in the strata of the earth's 

 crust, more frequently than is recognized (say one-twelfth to one-fifteenth 

 as often as the marine), evidence of subaerial deposits. Such deposits, 

 to be sure, are liable to be subsequently destroyed by wave action, will be 

 relatively meager in bulk, and will not invite attention because of their 



68 Annual Report of the Geological Survey of Michigan for 1901, p. 219. 



67 Geological Survey of Ohio, vol. i, 1873, p. 603. 



68 See Grabau : Types of sedimentary overlap. Bulletin of the Geological Society of 

 America, vol. 17, 1906, p. 613. 



69 Edinburgh Geological Society, vol. vii, p. 203. 



^'^ Quoted from Royal Scottish Geographical Magazine, vol. iii, p. 75. 



