PETROLOGY IN AMERICA. 



1"^ HE material for study offered to the American 

 geologist is rich in many respects, and perhaps in 

 no branch richer than in petrology. The vast tracts of 

 Tertiary lavas along and to the west of the Rocky Mount- 

 ains, the peculiar igneous rocks on the east side of the 

 great watershed, the varied series of lavas, tuffs, and 

 intrusive masses in the Palaeozoic and older formations 

 of the Eastern states, the extensive areas of igneous and 

 other crystalline rocks in the Lake Superior region, the 

 Adirondacks, Canada, etc., all present many points of 

 interest, and much valuable work has already been done in 

 the description and study of these rocks. These results we 

 owe in large part to the United States Geological Survey 

 and that of Canada, and to various state surveys : Min- 

 nesota, New York, Arkansas, Texas, etc. Besides this 

 official work, systematic petrographic research has been 

 carried on at several universities and colleges, such as 

 Johns Hopkins, Columbia College, Yale, the University of 

 California, and others. It is proposed in this paper to 

 notice a few of the more interesting contributions from 

 American sources during the last two or three years ; but 

 in selecting from so large and varied a literature we are 

 compelled to confine ourselves to certain areas and certain 

 groups of rocks. 



Among the older formations much attention has been 

 •given to the massive basic igneous rocks so extensively 

 developed in some parts of Canada and the United States. 

 Adams (i) has given us a very complete account of the so- 

 called " anorthosites " or felspar-rocks which constitute the 

 chief bulk of what has been named the Norian formation of 

 Canada. They are undoubtedly plutonic rocks of the 

 gabbro family, characterised, however, by a great prepon- 

 derance of plagioclase felspar, usually labradorite. Next in 

 importance, though much subordinate, to the felspar are 

 augite and hypersthene, or in the Saguenay district olivine. 



