PROCEEDINGS OF THE ACADEMY AND AFFILIATED 



SOCIETIES 



THE GEOLOGICAL SOCIETY OF WASHINGTON 

 The 285th meetmg was held at the Cosmos Club, May 13, 1914. 



INFORMAL COMMUNICATIONS 



A harite deposit on Castle Island, Duncan Canal, Alaska: E. F. 



BURCHARD. 



Andradite and gedrite from a contact metamorphic zone in the Washing- 

 ton-Duquesne camp, Arizona: F. C. Schrader. 



A microscopic method of measuring the refractive indices of liquids: 

 Fred. E. Wright ( See this Journal 4 : 269-279. June 4, 1914.) 



REGULAR PROGRAM 



The oxidized zinc ores of Leadville, Colorado: G. F. Loughlin. No 

 abstract. (This paper will form a chapter in a monograph on the Lead- 

 ville district to be published by the U. S. Geological Survey.) 



So7ne geodetic evidence of isostasy: William Bowie. The speaker 

 gave a brief summary of the results of the investigations by the Coast 

 and Geodetic Survey during the last few years dealing with the figure of 

 the earth and with gravity reductions. He pointed out the great in- 

 crease in the accuracy of the results due to the introduction of the theory 

 of isostasy. By this means Hayford was able to determine the figure of 

 the earth with greater precision than had been obtained previously. 



Especial attention was called to an illustration in Hayford 's second 

 report on investigations concerning the figure of the earth in which are 

 shown areas of excess and deficiency of mass, as indicated by the deflec- 

 tion anomalies. It was stated that a gravity station was placed in or 

 near 13 of the 16 areas indicated and, in each case, the gravity results 

 agreed with those of the deflections. The results of the recent gravity 

 investigations prove conclusively that there is no definite relation be- 

 tween the character of the topography and the sign and size of the gravity 

 anomalies if the reduction is made by the Hayford method. If the 

 Bouguer or rigid method of reduction is used there is a decided relation 

 between the topography and the size and sign of the anomahes. 



The data at hand do not show whether regional or local distribution of 

 compensation is nearer the truth. The reduction by both methods of 

 the distribution has been made by the Coast and Geodetic Survey, but 

 the results are negative. 



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