FAMILIES OF ROCKS 



feet in length and a foot in greatest diameter. From 

 this maximum they ranged down to forms smaller than 

 a pipe-stem. Their habits are to be gathered only from 

 their structure and from the habits of their relations in 

 the present seas. Perhaps they floated, shell uppermost, 

 or crawled upon the bottom and preyed on a variety of 

 the weaker forms of life. There appear to have been 

 fewer worms, perhaps because the muddy and chalky 

 sea bottoms of the Ordovician period were less congenial 

 to them than the Cambrian sands. 



The changes in the structure of the earth's crust which 

 brought the Ordovician period to an end marked also the 

 beginning of the Silurian period. These changes affected 

 sometimes small areas and were very intense ; sometimes 

 they affected larger areas more slightly. It must not be 

 assumed, however, that these changes were necessarily 

 sudden or violent. In examining the rocks now, we see 

 merely the effects, and of these effects it is the more re- 

 markable alone which have survived the march of ages. 

 There was more water on the earth's surface then than 

 now ; and side by side with continuous storms of tropical 

 violence, it is extremely likely that volcanoes and earth- 

 quake movements were more frequent and more con- 

 siderable in their effects. The tide of movement by 

 material things may have been faster; and certain 

 it is that the land was now lifting itself up above the 

 shallow seas. Mountains were being built along the 

 coast-lines ; behind the coast-lines the continents were 

 shouldering their way upwards in large land areas. 

 North America began to show in this period the first 



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