EARTH'S CRUST — WASHINGTON. 303 



unusual rocks occur along this zone. This is marked with dashes 

 ( — ) on the map. The sodic areas just mentioned may be connected 

 with this. West of the Appalachians we find a few small sporadic 

 occurrences of peculiar rocks, high in potash and magnesia, as in 

 New York, Pennsylvania, Kentucky, and Arkansas, which seem to 

 be distinct from the preceding, and which may represent the great 

 body of magma that underlies this eastern part of the Mississippi 

 Valley. 



Around Lake Superior, in Minnesota, Wisconsin, and Michi- 

 gan, and probably extending into Canada to the north, is an area 

 of igneous rocks that are low in silica but high in lime and iron 

 oxides. To the last feature is due the importance of this region 

 for its very abundant iron ores. It is marked with x's on the map. 

 In the southern part of the Mississippi Valley, about the Ozark 

 uplift, are some small and as yet little-studied occurrences of 

 granitic rocks, which seem to form a distinct region. 



West of the Mississippi Valley the comagmatic relations are more 

 complex, as are the geological structures, but we can distinguish 

 some fairly Avell-defined comagmatic regions. One of the most 

 clearly marked is that which extends from, and possibly beyond, the 

 Canadian border through central Montana, where it is represented 

 by several volcanic centers described by Pirsson and others, into 

 Wyoming, and with patches that probably represent it in eastern 

 Colorado. These rocks are characterized by decidedly high alkalies, 

 and with potash generally dominating soda. The areas are marked 

 by +'s on the map. Covering the great plateau of Colorado, Utah, 

 and Nevada, with parts of Idaho and Wyoming (including the Yel- 

 lowstone Park) and probably in northern New Mexico and Arizona, 

 is a large and complex region, the rocks of which are decidedly of 

 average composition, distinctly high in silica, moderate lime and 

 alkalies, and low iron and magnesia. North, west, and south of this 

 is a rather ill-defined region, whose rocks are similar but somewhat 

 more calcic. The first is distinguished by small circles and the latter 

 by dots on the map. These regions need further study, and it is 

 doubtful if they should be treated separately. 



In southern Idaho and in Washington and Oregon are the very 

 extensive flow basalts of the Snake and Columbia Rivers, high in 

 lime and iron oxides, which resemble chemically the rocks of the 

 Lake Superior region and which are marked similarly on the map. 

 The true relationship of these to the surrounding regions is doubtful. 

 Along the Pacific coast, chiefly in California, but extending to the 

 north about as far as Puget Sound, there are indications of a nar- 

 row zone of decidedly sodic but rather highly silicic rocks. This 

 may extend south along the west coast of Mexico, and may there 

 be connected with the origin of the jadeite objects found in that 



