abstracts: geology 531 



phoria formation of Idaho, which corresponds to the upper part of the 

 Park City formation. In western Montana north of latitude 45° 30' 

 the overlying Triassic shaly limestone and red shale were removed by 

 the Jurassic erosion and the Ellis formation (Upper Jurassic) rests on 

 the partly eroded Phosphoria formation. R. W. Stone. 



GEOLOGY.— Ceo/og^y of northeastern Montana. Arthur J. Collier. 

 U. S. Geol. Survey Prof. Paper 120-B. Pp. 17-39, pis. 6, figs. 5. 

 1918. 

 Describes a large thinly settled region in northeast Montana which 

 is part of the Great Plains. The topography is discussed and forma- 

 tions from Cambrian to Jurassic exposed in Little Rocky Mountains 

 are described briefly. The Cretaceous, Tertiary, and Quaternary form- 

 ations are described more fully, and the geologic structure is explained. 

 Drainage diversion due to the invasion of ice during the glacial epoch 

 forms an interesting conclusion of the report. R. W. StonE. 



GEOLOGY. — A contribution to the geology of northeastern Texas and 

 southern Oklahoma. Lloyd William Stephenson. U. S. Geol. 

 Survey Prof. Paper 120-H. Pp. 129-163, pis. 14. 1918. 



This paper sets forth the present state of knowledge in the areal 

 mapping, in the interpretation of structure, and in correlation, and 

 indicates certain mappable units and structural features that have not 

 heretofore been recognized in the region in central and northeastern 

 Texas and southern Oklahoma known as the Black and Grand prai- 

 ries. The area lies near the northwestern border of the Gulf Coastal 

 Plain in northeastern Texas and southern Oklahoma, and is a dis- 

 sected coastal-plain upland ranging in altitude from about 530 feet 

 in the southeast to 850 feet in places in the northwest. The drain- 

 ageways of the area present many good examples of consequent, sub- 

 sequent, obsequent and perhaps other classes of streams. 



The area is underlain throughout its extent by strata of Cretaceous 

 age, which rest upon a buried, moderately smooth basement com- 

 posed of ancient rocks. The tilted peneplained surface of the base- 

 ment rocks dips to the south from the northern boundary at rates 

 estimated to range in different places from 50 to 70 feet or more to the 

 mile, and to the southeast from the western boundary at rates prob- 

 ably ranging from 40 to 50 feet to the mile. 



The basement rocks are separated from the overlying Cretaceous 

 deposits by an unconformity representing a long interval of geologic 

 time, including at least the Triassic and Jurassic periods and prob- 

 ably a considerable part of the Lower Cretaceous epoch. 



