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



into quartzites throiigli the operation of the process give it great geolog- 

 ical importance. 



Application to tpie Sylvania Sandstone 



Haying thus presented the criteria by which the seven main types of 

 sand grains^ Avith all the intermediate subtypes, may be recognized, it 

 will be of interest to make an application of the principles to a specific 

 case. The formation selected is the Sylvania sandstone, first very briefly 

 described by the early Michigan and Ohio surveys, and then more fully 

 by Orton^^ and the present writer.^-^ The formation was supposed for 

 many years to represent the Oriskany in the Lake Erie region, but is now 

 known to be of mid-Monroan (Silurian) age, separating the lower dolo- 

 mites from those of the upper division. In a bulletin about to be issued 

 by the Michigan Geological Survey, treating of the Monroe Formation, 

 prepared by Prof. A. W. Grabau and the writer, the results of detailed 

 studies of this bed will be presented. Only those features need be men- 

 tioned here which throw light on the origin of the granules and the 

 method of their accumulation. The sandrock is a remarkably pure, in- 

 coherent, snow-white aggregation of quartz granules in crj^stalline condi- 

 tion. Practically the only other minerals present have been secondarily 

 introduced, such as *calcite, dolomite, iron oxide, and additional silica, 

 and the sand when washed can be utilized in the manufacture of high- 

 grade glass. The granules are imder a millimeter in diameter, and in 

 the case of 14 samples studied at the Michigan Agricultural College 

 averaged .2 to A millimeter. In certain samples the assorting is sur- 

 prisingly perfect (see figure 6, plate 47), and in all cases the granules 

 are rounded, continuing down to those under .1 millimeter (see figures 

 3 and 4, plate 47). 



From this much of the description, by a process of elimination we 

 may begin to locate the type of sand Avith which Ave are dealing. On the 

 basis of the form and assortment of the granules we may at once rule out 

 the glacial, residual, and much of the volcanic types, Avhile on the basis 

 of composition and crystalline condition the organic and remainder of 

 the volcanic would be stricken out. Man}^ of the grains show secondary 

 enlargement (figure 6, plate 46), Avith perfect crystal facets and sharp 

 edges, giving the sand a fresh, sparkling appearance. There is no evi- 

 dence of erosion subsequent to the deposition of the secondary silica, and 

 Avhen embedded in a dolomite matrix, as in the case of one bed, the silica 



«* Ohio Geological Survey, vol. vi, 1888, p. 18. Also vol. vii, 1893, p. 17. 

 ^■' Geological Survey of Michigan, vol. vii, part I, 1900, p. 53. 



