OF VOLCANIC ROCKS. 89 



paratively inconsiderable over this area ; yet it is, in its totality, of greater import for 

 the configuration of the surface of the planet than the production of the mountainous 

 and hilly regions mentioned. For it had the effect of connecting regions which were 

 before separated by the sea, and of increasing thereby the area of continents in cer- 

 tain directions. It is well known that the sea, which in the beginning of the volcanic 

 era extended in the valley of the Danube to above Vienna, bathed both slopes of the 

 Carpathians, and reached eastward to the foot of the highlands of Central Asia, has 

 since then considerably retired, and that the secular rise of northern Africa, Arabia, 

 and Asia Minor has, also in these countries, enlarged the continental areas. 



So much time has elapsed, in most countries, since the main phases of the vol- 

 canic era, that the influence of the events connected with them upon elevation and 

 subsidence has probably relaxed. It appears that portions of the areas of elevation 

 have suffered a periodical subsidence, not only since that time, but also during the 

 volcanic era. But its aggregate amount having been in most of them less than that of 

 elevation, the final effect manifests itself generally as a rise ; though it appears that 

 in recent times some regions, chiefly those which are situated on the northern and 

 southern boundaries of the great area of continental elevation, are undergoing a more 

 marked subsidence. Even now, however, those countries chiefly are rising which 

 were distinguished by eruptive activity in the volcanic era. 







These few instances will suffice to show how great have been the changes in the 

 configuration of some portions of the surface of the globe during the relatively short 

 geological time which has elapsed since the commencement of the volcanic era, and how 

 much superior they appear to have been to those which had been going on during 

 preceding periods of much longer duration. Those facts which are known in relation 

 to this subject, allow to infer, that the changes of level were proceeding at an acceler- 

 ated rate, during, and probably also in the time next preceding, the volcanic era, and 

 that the elevation was limited to certain belts of great extent which were, in the 

 majority of cases, distinguished from neighboring regions by eruptive activity. But 

 this is not true for all cases, since there are some regions the elevation of which was 

 especially grand, and in which very few rocks, or none at all, were ejected. In ex- 

 amining into the causes of the connection of elevation with the other events of the 

 volcanic era, we must therefore keep in view the distinction of those cases where it was 

 attended, and those where it was not attended, by eruptive activity. 



As regards the first cases, we may refer for their explanation to our previous 

 theoretical considerations of the causes and effects of the formation of fractures. If 

 the increase of volume by the slow and perfect crystallization of viscous masses be- 

 low the crust was the cause of the rending of fissures, a slow rise must have preceded 

 this process. That this was so on the European continent, is verified by the fact, that 

 the Eocene strata had been elevated quite considerably when the first of those sedi- 

 ments, which can be proved to have been contemporaneous with eruptive activity, 

 were deposited. If then the relief from pressure caused the extensive crystallization, 

 at the depth to which the fissures reached, of masses which had been held before in a 

 viscous condition by the pressure itself, an accelerated rise would be the effect ; and if 



(127) 



