OF VOLCANIC ROCKS. 93 



Leaving aside these theoretical considerations, and comparing only the differ- 

 ent facts mentioned in regard to the changes of level which have taken place on the 

 surface of the globe since the inauguration of the volcanic era, we are forced to the 

 conclusion Unit they must be intimately connected, as regards their causes, with the 

 other phenomena by which this era was distinguished from preceding periods. Eleva- 

 tion and eruptive activity, even when locally not quite coincident, are coordinate effects 

 of the cooling of the globe ; but while the one is its immediate effect, the other results 

 from it only by the concurrence of other agencies, which by themselves alone would 

 have been incapable of producing results of such magnitude. As there are distinct 

 phases in the history of eruptive action, dependent on and marking the evolution of 

 the globe, so we may recognize different stages in the mode of manifestation of the 

 elevating forces. In those elevations which took place during the volcanic era, there 

 are certain peculiar features evident, among which may be mentioned : the increase in 

 altitude of those mountain ranges included in the volcanic belts ; the union into one 

 main chain of several smaller ridges, the axes of which are situated on a line ; the 

 lateral connection of parallel ranges into highlands ; the oblicpue connection of the 

 ends of main chains, the axes of which are situated on parallel lines, but are remote 

 from each other longitudinally, by broad belts of mountainous regions (of this 

 character is the connection of the Alps and the Himalaya) ; the elevation, final- 

 ly, above the level of the sea, of large areas which had been submerged before, 

 whereby distant mountain ranges were connected by extensive lowlands, the size 

 of the continents increased, and their outlines rendered more uniform. The mode 

 of these great changes of the configuration of the surface of the globe was probably 

 a repetition on a large scale of similar changes which had been going on in former 

 eras of eruptive activity. It is not our object here to develop them. But throughout 

 the course of these changes there appears to be conspicuous a tendency to connect, in 

 certain directions, what was before disconnected, to increase in size and render more 

 delinite certain areas of elevation, and to separate them from other areas of subsidence, 

 which appear to have been likewise increasing in extent. We may even recognize an 

 increase of this tendency during the volcanic era, as the phenomena connected with 

 the basaltic epoch have been far more widely and generally distributed over the areas of 

 volcanic belts than those of the preceding (propylitic and andesitic) epochs. There must 

 be certain general laws which regulate the directions in which the connection of discon- 

 nected parts takes place. Their knowledge is still very limited. The admirable manner 

 in which Dana has laid out the grand outlines of the arrangement of the islands of the Pa- 

 cific, has indicated the way by which we may look for the solution of this problem. If the 

 structure of the crystallized crust of the globe gives the most probable cause of the 

 definite directions which may be l-ecognized in the outlines of continents and chains of 

 islands, it should be borne in mind, that the direction of structural planes, if they 

 exist, must vary gradually in depth, in the same ratio with the chemical composition, 

 and that they may at the depth of andesitic compounds be different from what they 

 would be in the granitic depths. The singular way in which the connections by ele- 

 vation are effected, and which has been already partly pointed out by Dana, appears to 



q (131) 



