PRESIDENTS ADDRESS—-SECTION C. 97 
that the sedimentary rocks of the Alps have been squeezed 
laterally by the pressure on them of the gneiss and other rocks of 
the central chain. The addition of C. Darwin’s hypothesis of a 
series of small injections each, as a rule, hardening before the 
next took place, makes this explanation still more probable, and 
it also agrees with the fact that the thrust has always come from 
the central and most disturbed districts, and not from the flanks. 
Also, as we now know that granite is transformed into gneiss by 
pressure, the occurrence of a gneissic zone surrounding the 
granite and unconformable to the overlying sedimentary rocks is 
accounted for without supposing the presence of Archzan rocks 
wherever mountain uplifts have taken place. 
Again, the common occurrence of granite in the central axis of 
a mountain chain, and of basic volcanic rocks on one or both 
flanks, cannot be accidental, and is partly explained by this 
hypothesis, which supposes the withdrawal of the upper acidic 
magma to the axis of the mountains, leaving the crust on the 
flanks to rest upon more basic material. In the same way it 
explains why the rocks brought up by oceanic volcanoes are 
almost exclusively basic. 
If the hypothesis be correct, it would follow that mountain 
ranges need not have solid roots to support them, as supposed by 
Sir G. Airy and Mr. Fisher. These roots may be liquid, and 
held in their position by the curvature of the crust, although 
constantly changing a little in volume and producing oscillations 
of the surface. Fluid roots would account for all the phenomena 
of the plumb-line and of the pendulum equally as well as solid 
roots, but Mr. Fisher objects that if the molten magma rose up 
into the base of the range the increase of underground tempera- 
ture would be greater in mountainous regions instead of being, 
as it is, less. But the form and position of the isogeotherms near 
the surface is a very complicated matter, and certainly does not 
rest upon this one point alone. Probably the less rapid increase 
of underground temperature in mountains than in plains is due 
to the far greater facilities for the downward percolation of 
surface water in the former than in the latter regions; and this 
seems to accord better with the fact that hot springs are com- 
monly found in mountains. 
Conclusion.—Professor J. D. Dana has lately shown that there 
is a system in the feature-lines of the earth’s surface which is 
world-wide in its scope ; and, since these feature-lines have been 
developed with the progress of geological history, the system 
must have had its foundation at the earth’s genesis, and has been 
developed to full completion with its growth. Consequently, this 
system cannot be due to superficial causes, but must come 
primarily from systematic work within. Professor-G. H. Darwin 
has attributed this systematic work to the moon, which, as the 
crust of the earth solidified, raised wrinkles on its surface; and it 
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