58 THE STORY OF THE EARTH AND JiAN. 



plateaus, noting as we proceed such liints as can be 

 obtained with reference to the more extensive oceanic 

 spaces. 



Before the beginning of the age, both plateaus seem 

 to have been invaded by sandy and muddy sediments 

 charged at some periods and places Tvith magnesian 

 limestone; and these circumstances were not favour- 

 able to the existence or preservation of organic 

 remains. Such are the Potsdam and Calciferous 

 beds of America and the Tremadoc and Llandeilo 

 beds of England. The Potsdam and Tremadoc are by 

 their fossils included in the Cambrian, and may at least 

 be regarded as transition groups. It is further to be 

 observed, in the case of these beds, that if we begin 

 at the west side of Europe and proceed easterly, or 

 at the east side of America and proceed westerly, they 

 become progressively thinner, the greater amount of 

 material being deposited at the edges of the future 

 continents ; just as on the sides of a muddy tideway 

 the flats are higher, and the more coarse sediment de- 

 posited near the margin of the channel, and fine mud 

 is deposited at a greater distance and in thinner beds. 

 The cause, however, on the great scale of the Atlantic, 

 was somewhat different, ancient ridges determining 

 the border of the channel. This statement holds 

 good not only of these older beds, but of the whole 

 of the Silurian, and of the succeeding Devonian and 

 Carboniferous, all deposited on these same plateaus. 

 Thus, in the case of the Silurian in England and 

 Wales, the wh'ole series is more than 20,000 feet 



