68 
THE GEOLOOTST. 
and indeed most modem investigators, in the sweeping natnre of 
the power he attributes to aqueous 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 origra 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 difierences 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 shoiild be always borne in mind ra 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 pressur-e. Electricity itself, which accom- 
