730 FRANK A. WILDER 



limestone of the Colorado stage, in which Inoceramus labiatus is 

 found in great numbers. The Benton shales, while not so rich 

 in fossils as the limestones, contain Ostrea co?igesta, Prio7iocycliis 

 ■wyomi?ige?isis, and other species, none of which are brackish 

 water forms. They contain also some selenite, but, in view of the 

 fossil content of the shales, it is probable that the selenite was 

 not formed by precipitation from concentrated brine at the time 

 that the shales were laid down, but is due to subsequent chemical 

 reaction in which sulphuric acid, generated perhaps from iron 

 pyrites, converted part of the lime carbonate of the shales into 

 the sulphate. In barrenness of fossils, in color, and in associa- 

 tion with gypsum the red shales which accompany the Iowa gyp- 

 sum resemble the Permian of Kansas much more than they do 

 the Cretaceous shales of Iowa. The presence of chalk in Sac 

 county, close to what must have been the Cretaceous shore indi- 

 cates that for a time sediments from land were at a minimum, 

 and organic sediments unmixed with land waste were able to 

 accumulate near the shore. This would indicate an absence of 

 the barren surface usually attending aridity, or the absence of 

 elevation, or both, so that climatic conditions favoring deposits 

 of gypsum are not implied by the chalk of the Cretaceous. 

 Regions devoid of rainfall are characterized by wind storms of 

 great violence capable of transporting much earthy material as 

 dust and carrying it out to sea, where it would ultimately be 

 deposited. The arid regions of America are subject to brief but 

 violent rain storms, during which erosion is vigorous on the sur- 

 face barren of vegetation. Low land surfaces covered with an 

 abundant vegetation are most favorable for pure chemical and 

 organic accumulations in the neighboring seas. The great purity 

 of many gypsum deposits presents a difficulty for this very 

 reason, for land vegetation must have been limited during the 

 concentration of the sea water and conditions favorable for dust 

 storms seem likely to have prevailed. Microscopic examination 

 of the Iowa gypsum reveals particles of sand scattered through 

 the gypsum, probably by wind, but the total amount is small, 

 amounting to about i per cent, of the whole. 



The age of the great gypsum and salt deposits of the world 

 is shown below : 



