Dr. C. Ricketts—Changes in the Earth’s Crust. 51 
change of opinion had taken place; very many of those who had _ 
been led to consider that, in the words of Hutton, “the rivers them- 
selves had hollowed out their valleys,” being on the staff of the 
Geological Survey, whose occupation affords special opportunities 
for determining such questions; not that they ignored the effects of the 
waves and tide, but did not attribute to them results which the sea 
could not produce, occurring where the sea had not been. The 
author of “ Rain and Rivers” (the late Colonel George Greenwood), 
was the most persistent advocate of the views of Hutton and Playfair. 
This dashing cavalry officer renewed the assault again and again, 
never failing to attack any weak point exposed by his opponents. 
This discussion was decisive in determining that valleys are 
formed by rain and rivers, and that the materials, which once filled 
up the excavations made, had been carried down and deposited in the 
sea near their mouths; but there still remained to be considered the 
methods by which this is accomplished. If the crust of the earth 
were rigid, rivers, by bringing down the disintegrated materials, 
would simply fill up the sea near their mouths, and, judging from 
the vast amount which must, during prolonged periods, have been 
removed in the excavation of valleys, there would be formed level | 
plains or deltas extending over an immense number of square miles, 
through which the streams would flow. At the mouths of large 
rivers. and where the sediment derived from the excavation of the 
land is brought down by them, there is evidence in all cases that 
the weight of these accumulations in the bed of the sea, on deltas, 
and in bays, presses down the crust of the earth, and thus accommo- 
dates the subsequent accretion of materials. This result may be 
considered universal and capable of demonstration in strata of all 
ages, from the earliest Geological epoch to the present time.’ 
No expression is in more frequent use than that different 
formations have been laid down during a period of subsidence. 
«« Everywhere throughout the world,” says Prof. James Geikie, “we 
read the same tale of subsidence and accumulation, of upheaval and 
denudation. It has only very lately been generally recognized that 
the weight of the accumulation is the cause of subsidence; at all 
events when, in a consideration of the subject,’ opportunity was 
afforded, there was no attempt to controvert this opinion. The 
subject is a most important one, and, with its converse, that denuda- 
tion, by lessening the pressure on the earth’s crust, causes the land to 
rise, may justly be considered the Alpha and the Omega of physical 
geology, for to it must be attributed the great movements and 
changes that take place on the earth. 
These areas of deposition may subsequently enter into the structure 
of mountain-masses and form elevated ground, but, so far as building 
1 On Subsidence as the Effect of Accumulation, by C. Ricketts; Grou. Mac. 
Vol. [X. p. 119, 1872. 
2 Mountains: their Origin, Growth, and Decay. 
3 “ Nature’? for August 2nd, 1883, and subsequent numbers. In the number for 
August 30th, 1883, page 413, and October 4th, 1883. page 539, reference is directed 
to those who have taken the subject into consideration so far as known to myself. 
