CHARACTER AND TEXTURE OF THE SANDSTONE 245 



in Minnesota, Olmstead, and Fillmore counties, in the southern part of 

 the state, exhibiting much coarser grain than the Minneapolis-Saint Paul 

 area. They also remark a noticeably coarser development at the base* 

 of the formation at Cannon Falls and Xorthfield. 



Under the microscope, grains from the Saint Paul area vary from 

 .05 to 2 millimeters in diameter. Of these the larger grains are all 

 much worn and as a rule very perfectly rounded, while the smaller, 

 .1 millimeter and less, are strikingly more angular. This same relation- 

 ship is shown by the samples from Missouri, where, however, the grains 

 range as high as .6 millimeter in diameter and a greater proportion of 

 them are well rounded. The Illinois rock is described as composed f of 

 very uniform small round grains, as seen under the microscope. Speci- 

 mens examined from La Salle county show more than the average im- 

 purity of iron and clay and fine matter. 



Southern Wisconsin has a similar record. In Michigan the narrow 

 strip through the upper peninsula that has been correlated with this 

 formation J is made up of the grains that are described as angular and 

 associated in places with fragments of the preceding and destroyed edge of 

 the magnesian. 



Two samples from Iowa sent by Ira A. Williams, of Iowa State Col- 

 lege, show the greatest range of any received. Both are from the eastern 

 border of the state. In one the grains are nearly all large, some exceed- 

 ing 1 millimeter, and all are rounded and beautifully pitted. The other 

 is very fine grained, chiefly .05 to .1 millimeter in diameter, and also 

 well worn. Both are very pure. 



Only the larger grains in most of the samples are well rounded. Those 

 of .1 millimeter and over are all much worn. Commonly the smaller 

 grains, .05 millimeter and less in diameter, are strikingly more angular. 

 In only one case, an Iowa sample of very fine grain, are all grains worn. 

 In one sample from Missouri, although the grains show marked second- 

 ary enlargement, the original character does not depart from the average 

 type. 



The worn grains in all the samples are beautifully pitted in the manner 

 so often seen on wind-worn fragments. The agreement of the textural 

 character of this rock with the requirements of a wind transported de- 

 posit is at once apparent. Every grain yet observed by the writer from 

 this formation falls within the range of wind competence. According 

 to the researches of J. A. Udden, grains comparable to those of largest 



* Bull. Geol. Soc. Am., vol. 3, p. 351. 



t Geological Survey of Illinois, vol. v, p. 116. 



J Geology of Michigan, vol. I, pt. iii, pp. 56, 64. 72. 



XX — Bull. Geol. Soc. Am., Vol. 17. 1905 



