200 GEOLOGY. 



In Fig. 110, 7 is the same with 

 6, but represented as coiled up. 

 The fossils of these animals are 

 often found in this posture in the 

 rocks, showing that the envelop- 

 ing mud or sand, which after- 

 ward became stone, caught them 

 in this condition. The size of 

 Fig - m - these animals varied much, from 



the sixth of an inch to even two feet in length. 



288. Climate. The climate of the Silurian age, it is 

 supposed, was quite uniformly warm over the whole 

 earth. Mollusks are not apt to be profusely abundant 

 except in warm seas, but, as the life-record shows that 

 they were so in such localities as New York, it is plain 

 that tropical climes were more extensive than now. Be- 

 sides, the wide diffusion of the mollusks shows that the 

 temperature of the earth generally was more equal. That 

 the sun shone then with clearness much of the time, how- 

 ever it might have been in the Azoic age ( 271), is plain 

 from the eyes which the Creator gave to those numerous 

 inhabitants of the sea, the Trilobites. 



289. Alternate Subsidences and Elevations. During 

 the different periods of this age there must have been 

 many changes in the level of the land when different 

 strata were forming. Take a single example. When 

 the great limestone floor of the New York basins ( 278) 

 was being laid, the water must have been of the requi- 

 site depth for the corals, and crinoids, and mollusks that 

 flourished then. For this purpose there was a subsi- 

 dence of the land, and there is evidence that this move- 

 ment extended far beyond the locality of these basins. 

 For instance, the Green Mountain region, which had pre- 

 viously been dry land, was now wholly or in part sub- 

 merged. But when, after the floor was laid, the salty 

 deposits were made, there was an elevation of the land 

 that shallow waters might prevail for th.e ready evapora- 

 tion of the water and the consequent deposit of the salt. 



