2 ScJiuchert and Barrell — Revised Geologic 



what might be called the center of gravity of geological 

 research took place westward!}', until now, at its end, the 

 most productive activity is here in America. This is not due 

 to any greater ability on the part of American geologists, but 

 to the superiority of their opportunities. Dana has well said that 

 America is the type continent of the world. All geological 

 problems are expressed here with a clearness and a simplicity 

 not found elsewhere (1900). 



North America is the type continent, because of its 

 simplicity of geologic structure, not only throughout its vast 

 extent but as well throughout the geologic ages. The other 

 continent of the northern hemisphere, on the contrary, is more 

 complex in structure, since only in the course of time, through 

 the welding together of several land masses by orogenic 

 (mountain-making) forces, has Eurasia been formed. A typical 

 continent, Dana states, is " a body of land so large as to have 

 the typical basin-like form, — that is, independent mountain 

 chains on either side of a low interior" (1895). 



A great part of the northern half of North America has the 

 form of a depressed shield, and has been well named by Suess 

 the Canadian Shield (see fig. 1). Here in the rocks is revealed 

 nearly all of its pre-Cambrian history, events which took 

 an eternity to accomplish, and the details of which will always 

 remain far less clear than those of the southern half of the 

 continent. Though the geological history recorded in the sur- 

 face rocks of most of the United States and Mexico is far 

 shorter and later in time, the sediments are better preserved 

 and contain an abundance of fossils accessible to geologists. 

 To the north, and more especially to the south and west of the 

 shield, lie vast depression fields, or neutral and subpositive 

 areas, which have tended through the geologic ages to lie 

 slightly below sea-level. Because of this low level, the sea 

 has often spread over these fields and recorded there the post- 

 Proterozoic events. 



To the east of the southern depression field occur the 

 basal remnants of a mountain system, exposing a complex of 

 metamorphic and igneous rocks and showing that again and 

 again through geologic time majestic mountain ranges, studded 

 at times with volcanoes, have been raised above the present 

 basal structures. These mountains lay on the western side of 

 Appalachia, the eastern shore of which has in the course of 

 time sunk deeply into the abysses of the Atlantic ocean. 



The western depression field is a very wide one, bordered 

 on the west by another old land, Cascadia, which faced the 

 Pacific, but of this land little is as yet known. These latter 

 two regions were the scene of tremendous geologic activity, 

 the struggle for dominance between the continent and the 



