PRESIDENT’S ADDRESS—SECTION C. 87 
If depression by loading be true, it is evident that the crust 
must rest on a fluid which moves laterally, so that depression in 
one place is compensated by elevation in another; and this was 
clearly recognised by Sir J. Herschel, the originator of the 
theory. That areas of elevation and of depression lie alongside 
of each other was the opinion of C. Darwin, although he did not 
suppose that the depression was caused by loading. Messrs. 
Medlicott and Blandford have also pointed out that the great 
plain or depression of the Indus, Ganges, and Brahmaputra 
is probably contemporaneous with the elevation of the sub- 
Himalaya, but they also state that it is not nearly sufficient to 
cause that elevation. 
If the crust of the earth be floating in hydrostatic equilibrium 
on a fluid interior, as seems probable, “then alterations in vertical 
pressure, if sufficient, must produce movements ; and we should 
remember that as these alterations act continuously in one 
direction for long periods of time, on plastic materials, smaller 
changes than we imagine may possibly bring about movements. 
At the same time, it is certain that there are other and more 
powerful hypogene agents at work causing oscillations of the 
surface, and perhaps the formation of geosynclinals is the only 
important movement that can be attributed to denudation and 
deposition. 
.We have next to consider the effect produced by changes in 
temperature. Mr. Poulett Scrope has claimed to be the first to 
originate, in 1825, the idea that sedimentation would give rise to 
local increase in temperature ; but a perusal of his book, called 
“Considerations on Volcanoes,” shows this to be a mistake. He 
says that, as sedimentary rocks are worse conductors than crystal- 
line rocks, the heat of the interior would accumulate in “a 
subterranean mass of lava more rapidly than it can pass off to the 
outside of the globe through the solid crust of over-lying rocks, in 
consequence of their inferior density and conducting powers. It 
is obvious,” he says, “that the caloric will be concentrated in the 
lava and continually augment its temperature, particularly that 
of the lower strata, which are the nearest to the source of 
caloric.” He further thought that this increase of heat might 
melt a portion of the crust, and in the later editions of his work 
he says that the expansion of the surrounding unmelted rocks 
would force up an axial wedge of molten granite, which, in its 
turn, would give rise to horizontal compression and crushing. 
But his concentration of caloric is by no means obvious, and he 
does not make it clear how rocks, which are strongly compressed, 
can add to their own compression by pushing up matter from 
below. Indeed, the whole hypothesis appears to be impossible. 
The second part of Mr. Babbage’s often-quoted letter to Dr. 
Fitton, in 1834, explains his views on the elevation of continents 
and mountain ranges by the expansion of rocks when heated. 
