LIMESTONE STRATA. 65 



the atmosphere. These little creatures formed immense 

 masses of coral, and together with the " Encrinites " (having 

 stony frameworks, figs. 15 and 16), were the chief artificers 

 of that age, and well did they show what perseverance was 

 able to accomplish, for through the thousands of years of 

 quiet which now succeeded, they must have filled up nearly 

 all the existing oceans with their structures, causing the 

 water gradually to flow from its former beds into other 

 situations, and thus from time to time was changed the 

 position of both sea and land. The strata of mountain 

 limestone which now exist are the remains of what these 

 little polypi then produced, for there is scarcely a por- 

 tion of this kind of rock, but shows evidences of having 

 been, once, coral, madrapore, or some analogous structure 

 (fig. 17). This mountain limestone sometimes occurs 



FIG. 17. ENCRINITIC LIMESTONE. 



thousands of feet thick, and extends over vast districts 

 both in Europe and America. 



During this period, although there were no great dis- 

 turbances, capable of displacing the strata, yet there appears 

 to have been frequent small eruptions of volcanic matter 

 through the crevices of the lower strata, filling up all 

 their cracks and vacuities, and in many cases rising to 

 the surface, overflowing with basalt, toadstone, and other 



F 



