abstracts: geology 191 



erroneous assumptions as to the density of the surface materials and also 

 by a departure from a state of complete isostasy. 



The mean without regard to sign of the new-method anomalies at the 

 89 stations in the United States is only 0.017 dyne. An anomaly of 

 + 0.017 dyne would be produced by an excess of mass corresponding in 

 amount to a stratum about 570 feet thick of density 2.67 (the mean 

 surface density of the earth) with the station at the center of one surface 

 of the disk and the disk of indefinite extent. An anomaly of — 0.017 

 dyne would be produced similarly by a deficiency of mass corresponding 

 to a stratum about 570 feet thick. The gravity observations indicate, 

 therefore, that the isostatic compensation is everywhere so nearly com- 

 plete that the excesses and deficiencies of mass above the limiting depth 

 of compensation correspond upon an average to a stratum only 570 feet 

 thick. The average elevation of the surface of the ground in the United 

 States is about 2500 feet, more than four times 570 feet. 



The evidence furnished by the new method gravity anomalies in regard 

 to the location and extent of the continuous areas of excess or deficiency 

 of mass in the United States, that is, of under-compensation or of over- 

 compensation, confirms and supplements that given by the observed 

 deflections of the vertical previously considered and published by the 

 Coast and Geodetic Survey in the second publication on the Figure of 

 the Earth and Isostasy. W. B. 



GEOLOGY. — The Tertiary gravels of the Sierra Nevada of California. 

 Waldemar Lindgren. Professional Paper U. S. Geological Sur- 

 vey No. 73. Pp. 226, with maps, sections, and illustrations. 1911. 



This report attempts to trace the Tertiary history of the Sierra Nevada, 

 by an examination of the Tertiary gravels and volcanic flows which 

 cover a large part of their western slope. The gravels, as well known, 

 have had, and still have, great economic importance, altho at present 

 restrictions connected with the disposal of debris have put a stop to 

 most of the hydraulic mining. The report is really a summary and 

 includes observations made not only by the author, but by numerous 

 other members of the Geological Survey. 



In the early Tertiary age this region was a deeply eroded mountain 

 range, this erosion having occupied the larger part of the Cretaceous 

 period. Pauses in the erosion, when the topography had been reduced 

 to gentle outlines, permitted deep rock decay and promoted the libera- 

 tion of gold from its matrix. Renewed uplift quickened erosion and 

 facilitated concentration. These conditions continued thruout Cre- 



