90 RICHTHOFEN — NATURAL SYSTEM 
the repeated rupturing of the crust caused a repetition of this process, in the way in- 
dicated before, epochs of ‘an accelerated rise would alternate with those of a retarded 
elevation. This theory will explain why a very considerable rise, and of greater ex- 
tent than that which had preceded, attended the phenomena of the basaltic epoch. 
Although we must consider this subterranean process as the chief cause of con- 
tinental elevations, so far as they attended the phenomena of eruptive activity, it was 
certainly not their only cause. There can be no doubt that the process of metamor- 
phism must have promoted those changes. As far as we understand the conditions 
required for its production, it appears that they could not be created by any other 
known processes in a degree similar to that in which we must necessarily suppose them 
to have attended the ejection of rocks, whether its mode be that of massive eruption 
or of voleanic activity. Even on the surface there are no places where we can observe, 
at the present time, metamorphic processes of greater intensity than near the orifices 
of voleanoes, or in solfataras, or at the theaters of other processes attending or succeed- 
ing volcanic activity ; nor is there at any other places more evidence afforded of subter- 
ranean metamorphic processes. The latter will exceed those visible on the surface in 
nearly equal proportion with the increase of temperature and pressure in depth ; 
while we may conclude theoretically, that the neighborhood of fissures filled with 
heated substances from below would not only have a higher temperature than 
other portions of the crust, but also allow of a comparatively free circulation of water, 
on account of its shattered condition. In respect to metamorphic action, as in all 
other respects, the grandeur of the phenomena of massive eruptions would make us 
presuppose that they would have given rise to a far more extensive and intense meta- 
morphism than the insignificant processes of volcanic activity. The occurrence of 
ancient eruptive rocks in belts of highly metamorphosed sediments removes it almost 
beyond doubt that they are connected in the relation of cause and effect. It is assumed 
that metamorphic processes, on account of the entering of water into the composition 
of rocks, and their crystallization, must be attended by an increase of volume, and 
will have the mechanical effect of elevating the surface. 
We may thus obtain a few hints as to the causes of the changes of level which 
those portions of the surface of the globe which were the theater of eruptive activity 
in the voleanic era, have undergone since its inauguration. The agencies to which 
they were due must be even more intricate than the motions of the crust, ‘because the 
movement in one direction may be the resultant of several forces working in opposite 
directions. Increase of volume by crystallization below the crust, in those regions 
where the formation of fractures relieved the masses from pressure, is probably 
the most potent of these agencies. Metamorphism would work in the same direction, 
while loss of heat would cause subsidence. All these agencies have their first and 
common cause in the formation of fractures by an upward tension, and all other 
phenomena of vulcanism are the immediate or mediate effects of that same cause, 
which itself results from the gradual cooling of the globe. 
There remain those cases to be considered where some of the most prominent 
mountain ranges, in the structure of which volcanic rocks take no part, were elevated 
during the voleanic era, at a much greater ratio even than those in which these rocks 
(128) : 
