REVIEWS 603 



fully studied here than in any other locality, and the paper deals mainly with this 

 fauna. The molluscan fauna is found to point to comparatively dry, upland, terres- 

 trial conditions during loess deposition, such as exist over the greater part of Iowa 

 today. 



Udden, J. A. Geology of Louisa County. Iowa Geol. Survey, Vol. XI, 



pp. 55-126, 1901. 



The physiography is discussed on pp. 60-65 ; wells and water supply, on pp. 

 96-100, 125, 126; Pleistocene deposits, on pp. 101-14; clay industries, on pp. 

 119, 120; drift gas, on pp. 121-24; soils, on p. 125. In the discussion of drainage 

 attention is called to divides which stand at the very brow of the west bluff of the 

 Mississippi River, and of part of the Iowa River. It is thought that these large 

 streams have widened their valleys sufficiently to reach divides separating them from 

 small streams to the west. A contour map shows the altitude of the rock floor and 

 levels reached by wells that did not strike rock. The rock surface of an upland in the 

 southwest part of the county stands 650 to 720 feet above tide, while that of a lowland 

 which occupies the remainder of the county averages scarcely more than 400 feet. The 

 drift probably averages 300 feet in depth in the townships east of Iowa River, and 

 ranges from 20 to 200 feet in townships west of the river. 



Three sheets of bowlder clay are recognized — the pre-Kansan, Kansan, and 

 Illinoian. The youngest, or Illinoian, covers all of the county except a narrow strip 

 on its western edge. The pre-Kansan drift contains much wood, fragments of coal, 

 and about twice as many fragments of local rocks as the higher drift sheets. Of the 

 crystalline rocks it contains more greenstone, hornblende rock, and schist than are 

 found in either of the other drift sheets, and a smaller proportion of dolomitic lime- 

 stone, and of rocks common in the Kewenawan. This lowest drift sheet is poorly 

 represented on the upland, but is a heavy deposit on the lowland tract and in pre- 

 glacial valleys that traverse the upland. Between the pre-Kansan and the Kansan 

 the Aftonian soil and a sand deposit have been found. The Kansan drift is jointed, 

 with oxidized seams in the joints, and contains diabase, granite, and Kewenawan 

 rocks, in larger amounts than the pre-Kansan. The Illinoian drift contains a large 

 number of dolomitic rocks brought from neighboring outcrops to the east, and a com- 

 paratively few crystalline rocks. A few exposures of Yarmouth soil separating the 

 Illinoian from the Kansan drift were noted, and a large number of exposures of the 

 Sangamon soil between the Illinoian drift and the overlying loess. 



Natural gas obtained near Letts is apparently from the Aftonian sand and soil 

 horizon between the Kansan and pre-Kansan tills. The gas is probably partly from 

 this soil, and partly from bituniinous material in the pre-Kansan drift, which may 

 include fragments of Sweetland Creek shale. The gas is in the upper part of the sand 

 only, the lower part being filled with water. In fifteen different wells the pressure 

 ranges from 4 to 10.5 pounds. The head of ground water is thought to be the princi- 

 pal factor which determines pressure. The depth of eight wells reported is 80 to 126 

 feet. 



Udden, J. A. Geology of Pottawattamie County. Iowa Geol. Survey, Vol. 

 XI, pp. 199-277, 1901. 



The physiography is discussed on pp. 203-16 ; well records and water supply, on 

 pp. 243-48,273-76: Pleistocene deposits, on pp. 248-67; ice scorings, on pp.268, 

 269; clays, on pp. 270-72, while the remainder of the report is devoted to hard-rock 



