302 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 1020. 



perhaps forked, or be evident only as small, separate, and apparently 

 structurally unconnected occurrences of similar rocks. 



Although some of the characters of any given region may be most 

 evidently recognizable by the mineral features, such as the color of 

 the pyroxenes or the peculiarities of the feldspars, yet these are all 

 dependent on the prominent chemical characters of the magma, so 

 that the chemical characters constitute the fundamental basis of dis- 

 tinction and characterization. In order to show the reader how, and 

 in how far, the chemical characters of various portions of the earth's 

 crust may differ, it will be well to note very briefly some of the best- 

 known comagmatic regions of the earth, stating only their most 

 prominent chemical features and omitting all details. 



Fig. 2. — Comagmatic regions of the United States. 



In the United States we find a long zone of disconnected areas 

 whose rocks are dominantly sodic. This zone apparently begins in 

 southwest Greenland, appears as a group of very similar small areas 

 in Ontario, Quebec, and New England (the so-called Novanglian 

 region), appears in New Jersey, Virginia, probably North Carolina, 

 in Arkansas, and finally as several areas in Texas. It is apparently 

 continued south in Tamaulipas in Mexico ; and what may be a con- 

 tinuation of it appears in some of the Antilles, in Brazil, and as 

 far south as Paraguay. These areas in the United States are marked 

 solid black in figure 2. The large " Canadian shield " around Hud- 

 son's Bay forms another region, which is dominantly calcic (anortho- 

 sites) , marked with v's on the map. Along the Appalachian uplift, 

 and probably extending into Maine, is another region, the rocks of 

 which are characteristically rather sodic granites, though some very 



