266 CARNEGIE INSTITUTION OF WASHINGTON. 



depths as are obtainable by the driller. The age of the batholith is 

 not definitely known, but it is believed by students of Georgia 

 geology to be probably pre-Paleozoic, and certainly not later than 

 early Paleozoic.* Of the later geologic history all that is demon- 

 strated by the features of the locality is profound degradation, re- 

 sulting in the development of a broad peneplain. Nothing is known 

 in the vicinity of later orogenic or volcanic events, and the Creta- 

 ceous and Tertiary formations of the Coastal Plain are thought not 

 to have covered this area. So far as is known, the region is one 

 characterized by prolonged geologic quiet, and it has probably been 

 exempt, as far as any locality which might have been selected in 

 the United States, from physical and climatic accidents competent 

 to disturb the arrangement of subterranean temperatures. 



Ecommic Conditions. — While the selection of the Lithonia district' 

 for the proposed boring was made solely on considerations arising 

 from the scientific demands, attention was also given while on the 

 ground to economic considerations affecting the cost of the work. 

 One of the essentials in the use of the diamond-drill is a good sup- 

 ply of water. This can be readily secured in the final selection of 

 the precise site of the boring. The district is crossed by a railroad, 

 from which several spurs run to quarries, and a suitable site can be 

 found near one of these lines. No serious problems are connected 

 with the transportation of machinery and fuel. There is rail com- 

 munication with the neighboring city of Atlanta. 



ACCESSORY INVESTIGATIONS. 



In the planning of the boring no other instrument has been con- 

 sidered than the diamond-drill. The rock could probably be pene- 

 trated by the churn-drill at less cost, but the churn-drill, by grinding 

 the rock to sand, destroys its structure and makes it impossible to 

 be assured of the uniformity of its lithologic character. The dia- 

 mond drill, on the other hand, removing part of the rock in the 

 form of a core, preserves a continuous record of the character of rock 

 traversed. The core, moreover, permits the prosecution of other 

 investigations in addition to the thermal. The strength and other 

 physical properties of deeply buried granite are practically unknown, 

 and the information which can be obtained as to these may prove of 

 importance to geophysics. 



It is at least worthy of suggestion that the boring could also be 

 utilized for the subterranean swinging of a specially constructed 



*Geol. Survey of Georgia, Bull. No. 9 A, 1902, p. 63. 



