( 95 ) 



Letter from M. Theodore Virlet to M. Arago, on the Phench 

 meiion of Dolomisation^ and the Transformation of Rocks in 

 general. 



I HAVE just read in a journal an account of the discussion 

 which took place at the meeting of the Academy of Sciences, on 

 the 12th October, regarding M. de Buch's theory of dolomisation 

 —a theory which must be allowed to be bold and ingenious, if 

 reference be made to the period in which it has been advanced. 

 It is known that I am far from acquiescing in all the opinions of 

 this celebrated geologist ; but as I did not hesitate, at a time 

 when comparatively unacquainted with the science, to combat 

 such of his opinions as I could not admit, I now think it due to 

 the well known independence of my character, to support a fact 

 advanced by him, which has been disputed, and which, besides, 

 has direct reference to a question which has occupied much of 

 my attention, viz. the transformation of rocks in general. This 

 is one of the newest and most important questions of geology, 

 and it ought to afford us the means of making rapid progress in 

 the study of the composition of rocks, and lead to the solution 

 of a multitude of facts hitherto regarded as inexplicable. 



Some years ago, in describing to the Geological Society of 

 France, the modifications occurring in a bed of hematitic iron, 

 which I had an opportunity of observing near Sargans, in the 

 Canton of St Gall, Switzerland, I was led, by the recollection of 

 numerous analogical facts falling under my own observation, 

 and what I have mentioned in my account of the Geology of 

 Greece, to consider the phenomena of the transformation of rocks 

 under two different points of view, and to divide the modified 

 rocks into two very distinct classes. 



1. Such as have been modified, whether by the prolonged 

 action of heat, or by that of electrochemical agents, or by both 

 of these causes united, which have changed the combinations or 

 primitive arrangement of the molecules in relation to each other. 



2. Rocks which have been modified by chemical actions and 

 reactions, with the assistance of foreign- agents (such as the gases), 

 which have acted directly upon them, and changed their primi- 

 tive nature. It is in this class that dolomite ought naturally to 

 be arranged. 



