ELEMENTS IN IGNEOUS ROCKS—WASHINGTON. 289 
and other rocks containing nephelite, peculiar hornblendes, and other 
minerals characteristic of such magmas. This zone includes areas in 
Quebec, New England, New Jersey, Arkansas, Texas, extends into 
eastern Mexico and the West Indies, and probably as far south as 
Brazil and Paraguay. Parallel with this, but usually farther inland, 
is a second zone of areas of rocks which are low in silica and the 
alkalies, but high in lime and iron. This starts in the great anortho- 
site area of eastern Canada and Labrador, appears in New England, 
the Adirondacks, Delaware, Maryland, and extends to Georgia and 
possibly farther south. The rocks of this region are typically gab- 
bros, diabases, and pyroxenites, dunite and other peridotites, with 
some granites high in lime, and are often accompanied by very basic 
ores and other products of differentiation which are very rich in 
iron and titanium. Farther inland and west of the Appalachian 
Range is another belt, less well defined but apparently in general 
parallel to the others, of widely isolated small occurrences of peculiar 
peridotites and other rocks, low in silica and very high in magnesia 
and iron, with little lime or soda but much potash. This last feature 
gives rise to the common presence of peculiar micas, which dis- 
tinguish these peridotites mineralogically from those of the preceding 
region. These areas occur in Quebec, New York, Pennsylvania, Ken- 
tucky, Arkansas, and probably still farther south. 
Passing over the broad central part of the continent, where igneous 
rocks are very sparingly present, we find a province east of the Rocky 
Mountains which is characterized by high alkalies, especially potash, 
so that the usually rare mineral leucite is here quite common. This 
region is best known in Montana, Wyoming, and Colorado, and may 
possibly extend into western Texas. In the region of the Rocky 
Mountains and the cordilleras generally the occurrences of igneous 
rocks are so numerous and the relations so complex that it is some- 
what difficult to unravel the various petrographic provinces. As a 
whole, however, the igneous rocks of this part of the continent seem 
to belong to one very extensive province, which is continued into 
Alaska on the north and along the Andes to the south. In general 
chemical character the rocks show rather low alkalies, with more 
soda than potash, rather high lime, and but moderate amounts of 
iron and magnesia, leading to the abundance of such ordinary rocks 
as feldspar-basalts, andesites, dacites, and some rhyolites. There is 
some evidence that the province as a whole may be divisible into 
several subordinate districts, but it is noteworthy that rocks so high 
in soda or potash as to contain nephelite or leucite are practically 
unknown west of the Rocky Mountains. There are also indications of 
what may be a distinct region along the coast ranges which is char- 
acterized by high soda and generally high silica, but this demands 
further investigation. 
