170 Mr. A. Gages on a Method of Obso-vntion 



thing of their origin, mode of formation, or intimate structure. 

 The various reactions to which we must liave recourse give us 

 the elements of wliich rocks are composed, but usually in a state 

 of combination wholly different from that in which they previ- 

 ously existed. Chemical analysis may enable us to form per- 

 fectly clear conceptions about the nature of definite mineral 

 compounds, but by its aid alone we could not hope to arrive at 

 any very certain results as to the character of the modifications 

 which such compounds might undergo in time. If this be true 

 of simple minerals, how much more so must it be of such hetero- 

 geneous substances as the majority of rocks, especially those of 

 metamorphic origin. 



The mechanical processes employed in the preparation of 

 fragments of rocks for analysis, as well as some of the chemical 

 operations to which the mechanically prepared substance must 

 be submitted, destroy the peculiar structural arrangement of the 

 rock, and intimately mingle different constituent minerals, or the 

 altered and unaltered part of the same mineral. It is only by 

 a series of comparative experiments, varied in every possible way, 

 that we could hope to solve the problem of the genesis of many 

 minerals and rocks, but particularly of the class known as me- 

 tamorphic. 



The simple action of acids and other dissolvents on many 

 rocks, removing from them certain parts and leaving others ex- 

 posed to view, affords us the opportunity of making sxich a series 

 of comparative experiments as may often enable us to discover 

 their mode of formation, and the character and extent of the 

 alteration they may have suffei'ed. It is important to remark 

 that the mechanical state of the substances to be acted on is not 

 an indifferent element in experiments of this kind ; the chemical 

 result will of coui'se be the same, whether the substances to be 

 acted upon be in the form of powders, of laminse more or less 

 fine, of rock fragments, or of crystals cut in the direction of some 

 of their cleavage planes ; but the true interpretation of the several 

 phsenomena observed will be essentially different according to the 

 geological origin of the substances under investigation. In sup- 

 port of this proposition I may allude to some examples lately 

 supplied by experiments which I have made, and of which the 

 following notice contains an outline. In carrying out these ex- 

 periments, I have paid special attention to the skeleton which 

 results from the action of acids upon thin laminae of rocks or in- 

 dividual crystals. 



One of the best examples of the value of this mode of exami- 

 nation by acids was afforded by a fibrous dolomite, found near 

 Miask in the Ural Mountains. The analysis of the mineral made 

 in the ordinary way gave a quantity of lime, magnesia, and silica, 



