Dr. CO. Ricketts—Oscillation of the Earth’s Crust. 305 
ing indications of ancient land surfaces now situated at great depths. 
It is more than probable that the amount of deposits, so far as it has 
been proved by borings, represents but a moderate proportion of 
their entire thickness. 
Beyond the alluvial plains or deltas, estuaries or bays are very 
generally formed, being the extensions or prolongations of depressed 
valleys, into which sea-water enters. Rivers bring down an amount | 
of debris immensely greater than can he disposed of by their over- 
flow during floods upon deltoid plains. The amount removed, as 
indicated by the spaces left between the flanks of valleys, and 
carried into the sea, would, if there were no subsidence, form very 
extensive plains where there is now sea, and would extend in some 
cases to hundreds, it may be even to thousands of miles beyond the 
present deltas; whereas, notwithstanding the great accumulations, 
deltas are by subsidence transformed into bays, and instead of 
shallow water the sea becomes, at a few miles from the mouth of the 
river, unfathomable by the ordinary deep-sea line. Lyell has 
directed attention to the rapid increase of depth, from shallow water 
at the mouth of the Mississippi, to 95 fathoms at a distance of 12 
miles; 144 fathoms at 20 miles; and 452 fathoms at 52 miles.’ 
The inference to be drawn from the occurrence of subsidence near 
the mouths of great rivers appears to be, that where there is the 
greatest amount of accumulation, there subsidence has occurred to 
the greatest extent, progressing more rapidly than the filling up of 
the areas by sedimentary deposits. | 
It is a remarkable coincidence that subsidence of land, when 
covered with heavy accumulations of perennial snow, should be 
a phenomenon of general occurrence; it prevailed in the northern 
hemisphere during former periods over immense districts, but is 
restricted to a limited area (Greenland) at the present time. All 
writers on glacial geology recognize this progressive submersion of 
the land during what is called the ‘Glacial Period,’ when extensive 
districts both in Europe and North America, now fertile and luxu- 
riant, were buried under a thick covering of snow; and the bays and 
seas in their vicinity were packed with icebergs and floating ice; 
also that a partial re-elevation occurred at its termination. This 
subsidence may be chiefly ascribed to the weight of the snow heaped 
upon the land, and in part to the clay which, in muddy streams, 
issued from beneath the glaciers, and was deposited in a sea covered 
with bergs and pack-ice bearing rock fragments, which they had 
carried from many distant localities. The melting of the floating ice 
caused these erratic pebbles, etc., to be dropped into this clay, and 
thus become an integral portion of the Boulder-clay. The combined 
pressure of the snow upon the ground, and the Boulder-clay upon 
the bed of the sea, weighed down the crust of the earth. It was 
again raised to a considerable extent upon the return of a more 
genial climate, which relieved the land of its load of ice and snow.’ 
1 Principles, vol. i. chap. xix. 
2 President’s Address, by Charles Ricketts, Proc. Liverpool Geol. Soc. Session 
1871-72; also an abstract of the same, Grou. Mae. Vol. IX. 1872, p. 119. 
DECADE II.—VOL. X.—NO. VII. 20 
