68 THE GEOLOGIST. 



and indeed most modem investigators, in tlie sweeping nature of 

 the power lie attributes to aqneons action. He may, in some degree, 

 exaggerate this ; but it is useless to close our eyes to the evidences 

 of the great revolution in opinion, which has recently taken place on 

 the Continent on the subject of the origin of rocks. The doctrine 

 of the force of pure igneous causes, such as students will find in 

 almost every English text-book, has now only a minority of sup- 

 porters in Germany and France. Wide differences of opinion, how- 

 ever, yet exist, which may be expected to give rise to prolonged 

 investigations and discussions. 



The following epitome of some of M. Delesse's more general views 

 will give a notion as to the mixed nature of the causes to which 

 rocks owe their origin : — 



" The problem of the origin of eruptive rocks is one of the most 

 complex in geology, and has given rise to interminable discussions, 

 in which the most opposite systems have seemed in turns to triumph. 

 These revulsions in opinion, sometimes very sudden, are to be attri- 

 buted to the exclusive importance attributed to one or the other 

 agents which have aided in the formation of rocks. In popular 

 language it is said that no two things can be more opposed than fire 

 and water ; but in nature no such antagonism exists, these two agents 

 often acting together. This should be always borne in mind in any 

 inquiries into the origin of rocks. When therefore we speak of an 

 igneous or an aqueous rock, we do not mean to restrict these terms 

 to their exact sense ; we must necessarily attribute to them a mean- 

 ing different from that of common language. When we say a rock 

 is of igneous origin, we do not necessarily say that it has been re- 

 duced to a state of fusion by heat alone : similarly, speaking of an 

 aqueous origin, we by no means limit the causes to the unique action 

 of water. In speaking, therefore, of a cause, it must always be un- 

 derstood that it is only referred to as the principal agent of forma- 

 tion. Of the causes referred to — heat, water, pressure, and molecular 

 action — one may have played a preponderant, but rarely an exclusive 

 part. Molecular action, also, it should be borne in mind, must only 

 be considered as a secondary cause, for it seems to have been induced 

 either by heat, water, or pressure. Electricity itself, which accom- 



