6o6 REVIEWS 



glacial, but their age is not decided. A blue-black till 30-70 feet thick, which occurs 

 in the lower part of the drift, is referred to the pre-Kansan stage of glaciation. Above 

 this in some cases is a yellow sand separating it from the overlying Kansan drift. 

 The sand is possibly of Aftonian age. A classification of the lots of 100 pebbles each 

 from each of the drift sheets showed the upper or Kansan to have more hornblendic 

 rocks, diabase, epidote, gabbro, red arkose, and chert, than the lower or pre-Kansan 

 drift. The lower drift is found to have more of the greenstone, schists, slate, jaspilite, 

 syenite, and Palezoic rocks than the Kansan drift sheet. A table shows to what dif- 

 ferent extent the several classes of rock suffered reduction in the glacial mill. The 

 leaching of the Kansan drift surface varies within short distances from 4 to 12 feet 

 in depth, because of the difference in texture or in drainage conditions. The thick- 

 ness of the loess which covers the Kansan drift is about 12 feet, except on slopes, 

 where it is 8 to 10 feet or less. Cross striae were observed near Perlee bearing S. 35° 

 E. and S. To" W., but no attempt is made to explain this rather striking and unex- 

 pected feature. 

 Wilder, F. A. Geology of Webster County. Iowa Geol. Survey, Vol. XII, 



pp. 63-235, 1902. 



The physiography is discussed on pp. 69-75 > the Pleistocene deposits, on pp. 

 128-38; wafer supply, on pp. 185, 186; and soils, on pp. 189-91. There is thus but a 

 small portion of this lengthy report devoted to Pleistocene features aud deposits. 

 Webster county lies within the limits of the Wisconsin ice invasion, and the author 

 calls attention to the slight amount of erosion and immaturity of drainage systems in 

 this part of the Wisconsin drift sheet. The Des Moines River is thought to be in a 

 pre-Wisconsin valley that was nearly filled with Wisconsin drift. The gravel terrace 

 150 feet above the river is thought to be of Wisconsin age. Attention is called to a 

 recessional moraine south of Webster county, and two others in the north part of the 

 county. There is also a weak one near Tara, previously noted by Upham. 



MISSOURI. 



Marbut, C. F. The Evolution of the Northern Part of the Lowlands oj 

 Southeastern Missouri. University of Missouri studies, Vol. I, No. 3, 63 

 pp. 6 Pis., 1902. 



After describing the lowlands, with their intervening hills and ridges and the 

 geologic formations of the region, the processes by which the lowlands were produced 

 are considered. It is concluded that the streams which eroded the lowlands were the 

 streams that now drain the region, and that the lowlands are so recent in their develop- 

 ment that no complete rearrangement of drainage can have taken place since their 

 erosion. It is thought that the Mississippi originally turned westward at Cape Girar- 

 deau and followed the course now occupied by the Advance lowland belt, and con- 

 temporaneously the Ohio eroded the Cairo lowland. The Ohio worked in loose sands 

 and clays, while the Mississippi worked chiefly in hard limestone. The Ohio was 

 therefore able in a given time to erode a much wider valley than the Mississippi. It 

 is also probable that it was less burdened by silt than the Mississippi, and in conse- 

 quence was able to bring its valley to a lower gradient. It is thought that the altitude 

 at Cairo may have been 60 feet lower than at Cape Girardeau, points similarly situated 

 on the parallel streams. This lower altitude of the Ohio gave it suffcient advantage 

 over the Mississippi to result in the capture of the latter stream. The capture is 

 thought to have first taken place through a gap between Benton Ridge and Crowley 



