20 THE STOET OF THE EARTH AND MAN. 



conditions in which they were deposited. It has 

 been already stated that the oldest wrinkles of the 

 crust of the globe take the direction of great circles 

 of the earth tangent to the polar circle, forming 

 north-east and south- west, and north-west and south- 

 east lines. To such lines are the great exposures of 

 Laurentian rock conformed, as may be well seen 

 from the map of North America (fig. 6), taken from 

 Dana, with some additions. The great angular 

 Laurentian belt is evidently the nucleus of the con- 

 tinent, and consists of two broad bands or ridges 

 meeting in the region of the great lakes. The 

 remaining exposures are parallel to these, and appear 

 to indicate a subordinate coast-line of comparatively 

 little elevation. It is known that these Laurentiau 

 exposures constitute the oldest part of the continent, 

 a part which was land before any of the rocks of the 

 shaded portion of the map were deposited in the 

 bed of the ocean all this shaded portion being 

 composed of rocks of various geological ages resting 

 on the older Laurentian. It is further to be observed 

 that the beds occurring in the Laurentian bands are 

 crumpled and folded in a most remarkable manner, 

 aud that these folds were impressed upon them before 

 the deposition of the rocks next in geological age. 



What then are these oldest rocks deposited by the 

 sea the firstborn of the reign of the waters ? They 

 are very different in their external aspect from the 

 silt and mud, the sand and gravel, and the shell 

 and coral rocks of the modern sea, or of the more 



