DISTRIBUTION OF CAMBRIAN ROCKS 369 



American. — In North America, Cambrian rocks are not exposed 

 at the surface over very large areas, being, for the most part, deeply 

 buried under later sediments. Their maximum thickness, so far as 

 known, does not exceed 12,000 feet, but this may be considerably 

 increased at the expense of the Algonkian. While not forming 

 extensive areas of the present surface, Cambrian strata are very 

 widely distributed over the continent, usually resting unconform- 

 ably upon the plicated and metamorphosed rocks of the Archaean 

 and Algonkian. These strata are found in the pre-Cambrian de- 

 pressions, from the Adirondacks to Newfoundland, and along the 

 flanks of the Appalachian uplift, from the St. Lawrence to Ala- 

 bama. They also fringe Archaean or Algonkian areas in other 

 regions, as in Wisconsin, Missouri, Texas, in the Rocky Mountain 

 chain, from Colorado to British Columbia, and in the mountains 

 of Nevada. Cambrian beds are exposed in the Colorado Canon, 

 and doubtless would be found throughout the larger part of the 

 continent, were the overlying beds stripped away. 



So far as they are accessible to observation, the Cambrian rocks 

 are chiefly such as are laid down in shallow water near shore, con- 

 glomerates, sandstones, shales (with some limestones), which are 

 ripple-marked in a way that betrays their shoal-water origin. 

 There are also some areas of deeper water accumulations, found in 

 the limestones of western Vermont, Nevada, and British Columbia. 



During Cambrian times the sea was slowly advancing over the 

 land in North America, and the geography of the continent was 

 very different at the close of the period from what it had been at 

 the beginning. In the Lower Cambrian the land areas are inferred 

 to have been somewhat as follows : First, there was the great 

 northern mass of crystalline Archaean and Algonkian rocks, but 

 this was probably much more extensive than the present expos- 

 ures of pre-Cambrian rocks would indicate. It probably covered 

 the whole Mississippi valley down to latitude 30 N. and extended 

 westward beyond the Rocky Mountains. Long, narrow strips 

 of land, alternating with narrow sounds, occupied part of New 

 England and the maritime provinces of Canada, while an Appa- 

 lachian land, whose western line is marked by the present Blue 



