AND ANALOGIES, OF ROCKS. 205 



earths, and probably with silica or lime, appears to 

 be also proved by certain appearances which take 

 place on breaking and drying some of these. In 

 marbles raised very wet from the quarry, a whitish 

 dusty surface soon follows, from the deposition of 

 the carbonat of lime ; and a similar deposition of 

 silica will account for that grey tarnish which is pro- 

 duced on pitchstones within a very few hours after 

 the specimens are broken from the rock ; during 

 which process of drying they also become far more 

 compact, or less tender. Thus the objection in ques- 

 tion falls to the ground ; were it even necessary that 

 the process of consolidation should be reserved for 

 that time at which the whole stratum was completed. 

 It is quite easy, on the contrary, to conceive that it 

 has, in many cases, proceeded gradually, even during 

 the deposition, as it actually does in the case of tra- 

 vertino. It is a very lax chemical view of such a 

 process, to imagine the necessity of a solvent entering 

 to deposit its contents, and then " excluding" itself. 



Of the different Rocks, and the Modes of their 

 Consolidation . 



Now, though a large portion of the strata of the 

 globe may have been brought into that form by this 

 last process, or by aqueous solution, and that a con- 

 siderable portion, at least, of the secondary ones pro- 

 bably owe their origin or consolidation to this cause, 

 there are many strata, particularly in the primary or 

 older series, to which it is impossible to apply it so as 

 to explain all the appearances which they present. 



There is nothing in the character of quartz rock, as 

 far as I have examined it, to prevent it from having 

 been consolidated to its present condition from the 

 long-continued application of an aqueous solution of 



