118 GEOLOGY OF ILLINOIS. 



a solution of the coloring matter held in chemical combination ; but in 

 most cases the color is caused by a formation over the surface of the 

 sihVious grains of sand. 



Distinct stratification exists in most of the outcrops, and even lines 

 of cross stratification are not rare. WHITNEY failed to notice wave 

 marks in the Wisconsin outcrops ; but there can be no mistake as to 

 the wave and ripple marks on the ferruginous layers of the Rock river 

 outcrops. Some of the large masses present abrupt and strong dips ; 

 but these are owing to local causes. No trace of organic life, either 

 plant or animal, has yet been observed in these sandstones. The era of 

 their deposition seems to have been a peculiar one. Great changes 

 must have taken place as it was ushered in and as it went out. 



A high axis of elevation runs along this heavy deposit. In either 

 direction from the river it dips away rapidly, and the overlying deposits 

 come on in quick succession. Rock river runs along this anticlinal axis, 

 having cut down almost or entirely through the formation. 



The heaviest outcrop of the deposit now under consideration, in the 

 whole area over which it is known, is the one along Rock river in Ogle 

 county. The formation is thin and wide-extended, embracing a super- 

 ficial extent in the northwest alone of more than four hundred miles in 

 length by over a hundred in width. At Starved Eock, on the Illinois 

 river, it is about one hundred and fifty feet thick. In Calhoun county 

 it outcrops in the Cap-au-Gres Bluffs to a thickness of perhaps eighty 

 feet. In Wisconsin and Minnesota its heaviest outcrops do not much 

 exceed one hundred feet in thickness. In Ogle county, however, we 

 think it reaches fully two hundred feet, and at the artesian well in Ste- 

 pheiison county it is perhaps considerably thicker. It is the identical 

 same rock known in the Missouri Reports as the Saccaroidal sandstone, 

 so extensively used in the manufacture of glass at Pittsburg. As ob- 

 served in Missouri, however, it is oftener of a light buff or brown color, 

 and has less of the white, pure silicious sand in its composition than 

 the same rock has in Illinois and further north. 



Geologists seem to be greatly in the dark as to the origin of this cu- 

 rious, interesting formation. 



Tlie Lower Magnesian Limestone. 



The Lower Magnesian limestone, or Calciferous sandstone of the 

 New York geologists, or its Western equivalent, comes, I think, to the 

 surface at several places in the bed of Rock river, between Oregon and 

 Grand DeTour. The floor of the river in many places along these high 

 sandstone bluffs, I am quite sure, is a harder, solider, and altogether 

 different rock. When doing field work in that part of the ground gone 



