OF NATURAL PHILOSOPHY. 145 



a great, height above the sea, has been attributed 

 tc several causes. By some it has been ascribed 

 to a plastic virtue in the soil ; by some, to ferment- 

 ation ; by some, to the influence of the celestial 

 bodies ; by some, to the casual passage of pilgrims 

 with their scallops; by some, to birds feeding on 

 shell-fish ; and by all modern geologists, with one 

 consent, to the life and death of real mollusca 

 at the bottom of the sea, and a subsequent 

 alteration of the relative level of the land and 

 sea. Of these, the plastic virtue and celestial 

 influence belong to the class of figments of fancy. 

 Casual transport by pilgrims is a real cause, and 

 might account for a few shells here and there 

 dropped on frequented passes, but is not extensive 

 enough for the purpose of explanation. Ferment- 

 ation, generally, is a real cause, so far as that 

 there is suck a thing; but it is not a real cause 

 of the production of a shell in a rock, since no 

 such thing was ever witnessed as one of its effects, 

 and rocks and stones do not ferment. On the other 

 hand, for a shell-fish dying at the bottom of the 

 sea to leave his shell in the mud, where it becomes 

 silted over and imbedded, happens daily ; and the 

 elevation of the bottom of the sea to become dry 

 land has really been witnessed so often, and on such 

 a scale, as to qualify it for a vera causa available in 

 sound philosophy. 



(139.) To take another instance, likewise drawn 

 from the same deservedly popular science : The 

 fact of a great change in the general climate of 

 large tracts of the globe, if not of the whole earth, 

 and of a diminution of general temperature, having 

 L 



