238 abstracts: economic geology 



but they are so unimportant that they have not been differentiated and 

 are here considered with the Quaternary. 



In the northwestern and the north-central parts of the area the 

 Archean system, consisting of granite and allied crystallines, outcrops in 

 a number of localities and everywhere lies within a few hundred feet of 

 the surface, but toward the south and east it slopes downward abruptly 

 and is found only at considerable depths. In the southwest the Sioux 

 quartzite, which is of Algonkian age, projects up through younger for- 

 mations and appears at the surface at numerous localities. 



In the east and south, where the granite is far below the surface, it 

 is overlain by a succession of indurated sandstones, shales, and lime- 

 stones, aggregating many hundreds of feet in thickness, at least the 

 upper part being of Paleozoic age. Throughout most of the western 

 part, and probably in isolated areas of the eastern part, of southern 

 Minnesota, Archean, Algonkian, and Paleozoic rocks are covered by 

 Cretaceous deposits consisting of soft, plastic shales and incoherent 

 sandstones which together attain a maximum thickness of at least 500 

 feet, though they are generally much thinner. A mantle of glacial 

 drift is spread out over all these formations and, with the exception of 

 the alluvium recently formed in stream valleys, is the youngest deposit 

 in the region. 



In regard to their importance in furnishing water supplies, the Pleis- 

 tocene ranks first and the Paleozoic second, while the Cretaceous and 

 Algonkian are of minor value, and the Archean is virtually destitute 

 of available supplies, everywhere marking the lower limit of water hori- 

 zons. H. D. McCaskey. 



ECONOMIC GEOLOGY.— The Lander and Salt Creek Oil Fields 

 Wyoming: The Lander oilfield, Fremont County. E. G. Woodruff. 

 The Salt Creek oil field, Natrona County. C. H. Wegemann. Bul- 

 letin U. S. Geological Survey No. 452. Pp. 87, with maps, sections, 

 and views. 1911. 

 Viewed as a whole the Lander oil field consists of a central highly 

 dissected ridge with moderately-trenched slightly-undulating plains on 

 either side. The topographic expression of the field is believed to have 

 resulted from the erosion of a plain composed of loose material that once 

 covered the area at a much higher altitude than the present surface and 

 sloped northwestward from the mountains toward the interior of the 

 Wind River Basin. 



The strata exposed at the surface in this field belong to the Trias- 

 sic (?), Jurassic, Cretaceous, Tertiary and Quaternary systems. Below 



