PI.ANS FOR OBTAINING SUBTERRANEAN TEMPERATURES. 265 



turbances created by their uplift can not be assumed to have disap- 

 peared and because their topographic ruggedness impHes irregularity 

 of isogeotherms. New England and the region of the Great L,akes 

 are unfitted because they were covered by the Pleistocene ice-sheet. 

 Attention was therefore restricted to the batholiths of the Southern 

 States. 



As to these I sought information from my colleagues on the 

 Geological Survey, finding the available information so full that I 

 was able to exclude some because associated with bold topography, 

 others because lacking uniformity of composition, and others because 

 traversed by joints. Of the localities not thus excluded the most 

 favorable appeared to be the Lithonia granite district in Georgia. 

 Of this I made a personal examination, and as it seemed peculiarly 

 favorable to the purpose, no other examinations were made. 



THE LITHONIA DISTRICT. 



In its general topographic character the Lithonia district is a 

 plain. The stream valleys, for the most part open, are excavated 

 to depths of 50 to 150 feet. A few rounded bosses of granite project 

 from 50 to 150 feet above the plain. The granite is surrounded 

 and in part overlain by schists, which appear to have originally 

 constituted the walls and cover of the batholithic chamber. The 

 continuity of the granite mass from outcrop to outcrop is inferred 

 from the close lithologic similarity found at all the outcrops. This 

 similarity includes not only composition, but a peculiar and unusual 

 structure, the granite having an imperfect schistosity, the planes of 

 which are everywhere contorted. It is therefore called by the State 

 Geological Survey contorted granite-gneiss. The rock is massive. 

 Only a few joints were observed, and these appeared to be occupied 

 by thin veins, and thereby sealed, so as not to affect materially the 

 continuity of the rock. The partings utilized in quarrying are 

 parallel to the surface and are usually not natural, but created by 

 blasting. They indicate a tendency toward exfoliation, which is 

 one of the characters of massive granite. In recent studies in the 

 Sierra Nevada I have found the tendency to develop partings par- 

 allel to the surface characteristic of massive rocks and absent from 

 rocks traversed by systems of joints. 



The extent of the granite body is not less than 10 miles in one 

 direction bj' 3 miles or more in the transverse direction. Uniformity 

 of character through such an area affords reasonable presumption 

 that uniformity will be found in the vertical direction to such 



