24 A. Tylor on Changes of the Sea-Level effected by 
gradual subsidence has occurred during that particular formation ; 
because the condition of equal depth of water during any deposit 
might be produced either by subsidence of the sea-bottom or ele- 
vation of the sea-level, or by both conjointly. 
In discussing these questions, the writer has not assumed that 
during gradual subsidences or gradual elevations, greater denuda- 
tions or depositions would occur than when the level of the land 
and sea-bottom was stationary; because it is not certain, either 
that during such gentle oscillations the forces that would produce 
denudation are sensibly diminished or increased, or that the rocks 
which are brought within the reach of denuding forces are neces- 
sarily more easily worn away than those which were previously 
exposed to the same influences. 
Parr I. 
It has long been acknowledged that the quantity of detritus 
annually carried into the ocean from various sources must dis- 
place an equal volume of water, and thus tend to raise the level 
of the sea. any years since it was estimated by an Italian that 
this change might amount to one foot in a thousand years. The 
general opinion on this subject has been, that the effects produced 
by the present supplies of detritus would be too minute to be 
perceptible, and on geological inquiries the ocean-level has been 
considered as permanent for all practical purposes.* I here pro- 
pose to offer the evidence of present denudation in certain coun- 
tries where careful observations have been made, in order to show, 
that, if such rapid destruction of land occurs in most localities, — 
then the operation of present physical causes must be amply suf- 
ficient to effect a perceptible alteration in the sea-level in a mod- 
erate space of time. ; 
‘The mere consideration of the number of cubic feet of detritus 
may be obtained by calculation from the published accounts of 
the quantity of mud annually abstracted from districts of known 
dimensions by their rivers. In this manner it is found that the 
Ganges would in about 1751 years, at its present annual rate, car- 
ry away from the land it drains (which is supposed to be about 
400,000 square miles) as much detritus as would cover that area 
to the depth of one foot, as the following calculation will show: 
Thus, 27,870,400 (superficial feet in a mile) x 400,000 = 
11,151,360,000,000, the number of superficial feet in the area of 
400,000 square miles drained by the Ganges. ‘The number 
* Manfredi. See Lyell’s Principles, edit. 1850, p. 270 and 542. 
