3G Dr. Mac Culloch on the Origin, Materials, 



and the solution of their calcareous matter, into masses of solid 

 rock. In this way, calcareous rocks are formed, partly by che- 

 mical agency, and partly by that of submarine animals. 



Where iron becomes converted from the metallic or oxidulous 

 state to that of rust, it becomes the cement of all the smaller 

 materials within its reach ; and thus sandstone is often formed 

 on sea-shores, in sand and gravel beds, and, very probably, to a 

 considerable extent in the noted ferruginous sand stratum of 

 England. 



Thus two modes of producing rocks, by the agency of water, 

 are demonstrated. It remains to inquire, what probability there 

 is, that the same agency can convert silica to that end, as we 

 cannot produce any instances so perfect, of its absolute action in 

 that way. 



The solubility of silica in water cannot be a matter of dis- 

 pute, however difficult it may be to effect its solution in our 

 laboratories. In my work on the Western Islands, I have pro- 

 duced nearly all the instances of this nature that are required for 

 the present purpose ; but I may here add to these, its actual so- 

 lution in the hot waters of Iceland and Italy, and the consequent 

 production of siliceous tufas and stalactites. To convert this 

 property to the present purpose, it is not requisite that the solu- 

 tion be very extensive, or very rapid. If we conceive this agent 

 acting for a long series of years in a mass of loose sand or 

 of clay, it is not difficult to see that the final result must be, in 

 the first instance, the formation of a sandstone, and, in the other, 

 probably, that of a schist. 



That this is the fact, in nature, is almost demonstrable, from 

 the frequent partial occurrence of sandstones in beds of loose 

 sand, and from the mixed chemical and mechanical texture of 

 almost all the solid sandstones. This effect, it is true, has some- 

 times been attributed to the action of heat ; but to adduce as an 

 agent that which cannot be shewn capable of producing a given 

 effect, while we are in possession of one that has the desired 

 power, is to abandon sound reasoning for the sake of maintain- 

 ing a species of fictitious analogy, which, after all, is not ne- 



