PETROLOGY IN AMERICA. 461 



more basic nature, which the author ascribes to processes 

 of differentiation in the gabro-magma during its cooling. 

 These include various peridotites and pyroxenites, or as 

 Bayley names them, to mark their affinities, " non-felspathic 

 gabbros ". There are olivine-pyroxene rocks, varying- to 

 pure oli vine-rocks ; pyroxene-aggregates ; pyroxene-magnetite 

 rocks ; and rocks containing up to 90 per cent, of titaniferous 

 magnetite. Analyses of this magnetite have yielded from 

 2 to 16 per cent, of titanic acid. Associated with these 

 various basic modifications of the gabbro are rocks differing 

 from the normal gabbro in possessing the granulitic struct- 

 ure. They differ also to some extent in mineralogical 

 constitution, hypersthene largely taking the place of the 

 olivine, while the augite is more or less replaced by biotite 

 and hornblende. 



It is well known that rocks of the gabbro family, closely 

 comparable with those of Canada and of the Lake Superior 

 region, are largely developed in the Adirondacks, in the 

 northern part of the state of New York. A large part of 

 these rocks are "anorthosites," composed mainly of labra- 

 dorite felspar with only subordinate augite and hornblende 

 and usually some red garnet, perhaps secondary. The 

 rocks are much affected by cataclastic structures. These 

 anorthosites form the heart of the mountain region, while 

 more basic gabbros constitute the smaller outlying intrusions 

 and minor portions of the main ridges. The latter rocks 

 have been described by Kemp, as they occur on the eastern 

 side of the mountains, along the western shore of Lake 

 Champlain (4). The felspar seems as before to be labra- 

 dorite : the other minerals are augite, hypersthene, titani- 

 ferous magnetite, and occasionally olivine. In places there 

 are bodies of iron-ore with ] 3 to 16 per cent, of titanic acid, 

 and these are regarded as representing an extremely basic 

 phase of the gabbro magma itself. 



These iron-ores are described in more detail by the 

 same author in a bulletin dealing with the geology of two 

 townships in Essex County, N.Y. (5). The ores are all 

 essentially of magnetite, but those of Westport township 

 are of little value on account of the considerable amount of 



