Reviews — American Geological Surveys, loiva. 223 



tinuous from the surrounding States, the valleys having been eroded 

 in Glacial Drift deposits. This plain averages 800 feet in height 

 above the ocean ; the lowest point in the south-east corner being 444 ft., 

 and the highest 1,200 feet, though more than a thousand miles from 

 the sea. • There are two systems of river drainage ; about one-third 

 of the streams flowing south-westerly into the Missouri Eiver, and 

 about two-thirds emptying south-easterly into the Mississippi Eiver. 

 Lakes are numerous, generally in the Drift, rarely rock-basins. 

 Frequently walls of boulders from two to ten feet high, and from 

 five to thirty feet wide, occur on the borders of these lakes. They 

 were called " Lake Eamparts" by the elder Hitchcock, and seem to 

 have been brought into their present position by the expansion of 

 the water in freezing. 



The Drift is common, having the composition of the ordinary 

 Boulder-clay, without stratification ; the largest surface-boulders 

 weigh fifty tons. The Drift of the north-west part of the State 

 contains proportionally more boulders, pebbles, and sand, while that 

 of other districts shows more clay and finely comminuted materials. 

 In the north-east part there is a "driftless region," bordering the 

 Mississippi 130 miles long, and about 2,700 square miles in area. In 

 this boulders are very rare, but there are some Drift-clays. It is a 

 part of the driftless area of over 30,000 square miles pointed out 

 by J. G. Percival and J. D. Whitney in the Geology of Wisconsin. 

 Immediately parallel to it on the west the boulders are unusually 

 numerous. The maximum thickness of the Drift is from 150 to 200 

 feet, and it is best developed along the dividing ridge. It is so abun- 

 dant in the north-west quarter of the State that the underlying forma- 

 tions are not represented upon the Geological Map which accompanies 

 the report, on the scale of twenty miles to the inch. The Drift is 

 universally of northern origin, and stride are found in only four out 

 of 102 counties. Near Burlington and generally their course is 

 S. 10°— 40° E., but near Council Bluffs they run S. 50° W. 

 The ice is thought to have been produced by glaciers. The "Bluff," 

 or deposit, overlies the Drift, and is confined to the Missouri valley. 

 It is part of a continuous band, more than 200 miles in length, and 

 nearly half as wide ; it is a sort of a terrace, and composed of Silica, 

 82-15 ; Iron, 3*89 ; Alumina, 6*7 ; Carbonate of Lime, 9-66 ; and 

 thus closely resembles the silt of the Missouri now forming. Pro- 

 fessor White thinks the material of the Bluff came from the calcareous 

 Cretaceous strata along the upper Missouri river. 



Dr. D. D. Owen was the first to publish the existence of Cretaceous 

 beds in Iowa, but Jules Marcou was the first to notice it away from 

 the immediate vicinity of Sioux city. The beds probably covered a 

 fourth part of the State, though but a few patches of it are repre- 

 sented upon the map. The rocks are incoherent sandstones and 

 chalky limestones. Eemains of Squaloid Selachians, Ptychodus, three 

 Teleostei, scales of a Lepidoganoid ; Inoceramus, Ostrea, and GloiuUna 

 comprise the fauna of these beds. The Iowa division of the Creta- 

 ceous is the south-east extension of the enormous Cretaceous area 

 described on the Upper Missouri by Meek and Hayden. 



