492 RECORDS 



Mr. Finlay then briefly discussed the distribution of similar 

 rocks in the various portions of the United States, and exhibited 

 a very instructive series of comparative charts of the chemical 

 composition of the rocks examined and those of allied groups, 

 the charts being constructed on the principles of the graphic 

 method devised by Professor Hobbs, as worked out by Mr. 

 Finlay. 



The second paper, that of Mr. Hill, also treated of Mexican 

 rocks, and the same geographical maps were employed to illus- 

 trate both papers. Little has been written about the coal bear- 

 ing rocks and their associated eruptives in the State of Sonora, 

 Mexico. The work done by Professor Dumble and his asso- 

 ciates has thrown considerable light on some of the problems. 



In the district investigated are representatives of nearly all the 

 formations from the Archaean granites to the Quarternary sands 

 and gravels. The most important division, however, is the 

 Triassic. The slates, sandstones, quartzites, etc., with coal 

 seams, make up the lower or Bananca division of the Triassic, 

 while an immense series of associated eruptives, including ande- 

 sites, dacites, tuffs, andesitic conglomerates, etc., is considered 

 the upper division. To the series of eruptives the name of Lista 

 Blanca has been given. The Lista Blanca has hitherto been 

 considered post-Cretaceous. 



In addition to the pre-Cretaceous eruptives, there are numer- 

 ous intrusives and flows of diorites, rhyolite and basalt, and in 

 one instance, trachite. It is probable that these are mostly of 

 Tertiary age. The diorites exert a very noticeable effect on the 

 formation of the ore bodies of the region. 



Specimens of all the eruptives were brought to New York 

 and studied by Mr. Hill in thin section under the microscope. 

 A series of chemical analyses of the type rocks was made and a 

 very clear relation established between the magmas of the dif- 

 ferent flows. The remainder of the paper dealt with details of 

 the petrographic characteristics of the rocks. 



Both papers were discussed by Professor Stevenson, Professor 

 Kemp, Dr. Julien, and Dr. White. 



Theodore G. White, 



Secretary. 



