ORBICULAR GABBRO-DIORITE 



295 



principal localities: the. "prune" or "pudding" granite of Craftsbury, 

 Vermont; the orbicular diorite at Rattlesnake Bar, El Dorado county, 

 Cahfornia; the granite from Clark's Peak, Medicine Bow Range, 

 Colorado; orbicular granite from Quonochontogue Beach, Rhode 

 Island; a spheroidal granite from near Charlevoix, Michigan, in the 

 northwest portion of the Lower Peninsula; a nodular granite from 

 from Pine Lake, Ontario; and the orbicular gabbro at Dehesa, San 

 Diego county, California. 



The nodules of spheroidal rocks are considered similar in some 

 respects .to the dark segregations, or "schlieren," so frequently 

 observed and described in granites. Like the spheroids, the dark 

 segregations are usually more basic in composition than the general 

 magma out of which they have segregated. A noteworthy exception 

 to this, however, is the nodular granite from Pine Lake, Ontario, 

 described by Adams,' the nodules of which are very appreciably 

 more acid than the granite matrix, although the analysis^ of the 

 matrix indicates a very acid granite. The schlieren of granites in 

 general are often of rounded outline, and at times show a sharp line 

 of demarkation from the inclosing matrix, but, as a rule, they display 

 no tendency to separate from the granite when broken. In many 

 cases the segregations (schlieren) and spheroids are quite similar in 

 mineral composition, but the dark segregations do not manifest the 

 concentric and radiating structures characteristic of the spheroidal 

 nodules. 



Last summer (1903), while engaged in field study of the granites 

 of North Carolina for the State Survey, the writer had occasion to 

 examine very carefully in the field an orbicular gabbro-diorite in 

 Davie county,^ and later to study, microscopically, thin sections cut 



Geology, Vol. Ill (1904), pp. 383-96; abstract, Science, N. S., Vol. XV (1902), 



P- 415- 



Some of the references given above on the European localities have not been 

 accessible to me. Those not accessible have been quoted from Professor Kemp's 

 paper in the Transactions of the New York Academy of Sciences, cited above; and 

 from Sir A. Geikie's Text Book of Geology, 1903, 4th ed., Vol. I, pp. 206, 224. 



1 "Nodular Granite from Pine Lake, Ontario," Bulletin of the Geological Society 

 of America, Vol. IX (1898), pp. 163-72. 



2 Ibid., p. 169. 



3 Knovi'ledge of the existence of this rock dates back many years, but, so far as I 

 am aware, a full description of it has not been published. Very brief mention of it 



