96 M. Virlet on the Transformatum of Rocks. 



The first manner of regarding the modification of rocks, which 

 I was the first to propose, allows me to explain how certain beds 

 placed in the midst of other beds, may be more modified than 

 the latter, or may even undergo a complete modification, without 

 the others, whether they were in contact, or even formed the 

 lower part of the same deposit, experiencing any sensible change 

 in their original state, and that without any of the beds being 

 confounded with each other. The opinion which I advance on 

 this subject, results as much from my own observations as from 

 the manner in which I regard the first sandy deposits as being 

 formed at the period when the waters began to condense on the 

 surface of the earth ; and although many may regard it as some« 

 what heretical, I have no doubt that it will soon be admitted by 

 all accurate observers, viz. That all stratified rocks, without ex-- 

 cepting gnelsSt the mica-slates, or clay-slates, S^c. have been 

 originally rocks of sediment, Jbrmed by mechanical aggre- 

 gatkyfi, and that they have acquired the crystalline characters 

 which noio distinguish them, by a series of modifications which 

 ihey have undergone posteriorly to their being deposited. 



It is conceived, on the contrary, according to the second kind 

 of modification of rocks, that, in the greater number of cases, all 

 the beds are confounded in such a manner as to present a single 

 mass without distinct stratification ; such, for example, as dolo- 

 mite, certain deposits of sandstone and clay transformed into 

 jaspers or trachytic porphyries, and other rocks which I have 

 often had occasion to enumerate ; for the chemical agents, by 

 penetrating across a certain number of beds, or even the entire 

 mass, have separated a part of the elements of the original 

 rocks and substituted others, or else have formed new combina- 

 tions, and finally united the whole mass of the deposit. It is to 

 these considerations that I wish chiefly to direct attention, as 

 they have reference to the phenomena of dolomisation. 



I do not dispute, I even admit, that there are dolomites which 

 should be called primitive, whatever may be their geological age ; 

 that is to say, which were the result of a series of simultaneous 

 deposits of carbonate of lime and of magnesia, for magnesia was 

 at least as abundant in nature as lime, particularly at the time 

 when the old deposits were formed. ^l[ie?>Q primitive dolomites, 

 however, always present a distinctive character in being regularly 



