ON THE THEOBIES OF ELEVATION AND EARTHQUAKES. 73 
The tendency of this mass would be to produce a subsidence of the surface 
on which the original deposition took place, and on which the whole sedi¬ 
mentary mass reposed. If the solid crust of the earth were sufficiently tliick, 
the depression would probably be insensible; but if the weight of the sedi¬ 
mentary mass acted oo a solid crust of large area, but not of a thickness 
greater than what we have already shown (art. 29.) to have been the pro¬ 
bable thickness of those portions which have been subjected to great angular 
displacements, we easily perceive that a slow but considerable subsidence 
would, under simple and probable conditions, be the neoeesary consequence. 
This reasoning would lead us to the conclusion that, in any proposed area, 
periods of great sedinientarr/ deposition had been periods of gradual subsi¬ 
dence. Now that such has been actually the case, we have, I conceive, the 
most conclusive and independent evidence. The admirable researches of 
Professor E. Forbes seem to establish beyond doubt, that comparatively few 
species of marine animals are ca])abIcof existing in the full exercise of their 
animal functions at depths exceeding 1000 or 1200 feet. Consequently we 
may conclude that during the deposition of a mass of futeilifcrous beds, 
having a thickness greater than lOOD or 1200 feet, there must have been a 
subsidence equal at least to the difference between the actual thickness of 
the stratified mass and that just mentioned ; aud if the stratification be con¬ 
formable throughout, we may also conclutlc that the dopresaion must have 
taken place, not by any paroxysmal movement, but by slow and gradual sub¬ 
sidence, a conclusion in exact accordance with that above eiianciated. 
Sir John Hersehel was tlie first, I think, to direct attention distinctly to 
the possible effect of a great mass of sedimentary deposits, in depressing the 
jwrlion of the earth’s crust on which it rested. lie also suggested the pos¬ 
sible influence of the depression of one portion of the crust io producing an 
elevation of a neighbouring portion; and, in fact, if both jiortions were 
superincumbent on the same continuous mass of fluid or semifluid matter, 
such elevation would probably be the necessary consequence of the neigh¬ 
bouring depression. I am not aware however how far geological observa¬ 
tions afford evidence of the synchronism of such opposite movements in 
adjoining areas. 
Another consequence of sodimontary deposition was suggested, I believe 
contemporaneoudy and indepemlently, by Sir John Hersehel and Mr. Bab¬ 
bage. The temperature of sedimentary matter at the period of its deposi¬ 
tion must be approximately that of the superficial temperature of the earth 
at the place of deposition, but the effVet will be to increase tlio temperature 
O'the mass beneath, by causing the rise of the interual isothermal surfaces. 
®oViacent fluid mass avouIU thus receive an acce.‘'sion of temperature 
which might again give to its expansive force sufticient energy to elevate 
and dislocate the superincumbent m.ass. The effectiveness of this cause in 
^dueing elevation, as well as the weight of sedimentary matter in pro¬ 
ducing subsidence, would raanifeallv lie increased where the thickness of the 
^ comparatively small. 
• It. Babbage has deduced from this rise of the isothermal surfaces, an 
explanation of the slow upheaval of large areas, by roferring it to the 
eolnmoar expansion that must result from the augmentation of temperature 
m the subjacent rocks. This deduction however appears to me inadmissi- 
In the first place, if this theory were true, a period of great sedimen- 
lary deposition in any assigned area would be a period of elevation, instead 
ot being (as we have above shown it must have been) a period of depres¬ 
sion, since the rise in the isothermal surfaces would necessarily be contem¬ 
poraneous with the process of deposition; and in the second place, the rate 
