THE PERMANENCE OF CONTINENTS. 
125 
words of Prof. Huxley, ‘ Surely there is evidence enough and 
to spare that the Cretaceous sea, inhabited by various forms, 
some of whose descendants Sir W. Thomson, as I believe justly, 
recognizes in the present deep-sea fauna, once extended from 
Britain over the greater part of central and southern Europe, 
North Africa and Western Asia to the Himalayas. In what 
possible sense can the change of level which has made dry land 
of, and sometimes mountain masses of, nine-tenths of this vast 
area, he said, to be “ in direct relation to the present ex- 
isting coast-lines.” That the abyssal plains were ever all 
elevated at once is certainly so improbable that it may justly 
he termed inconceivable ; hut there is nothing, so far as I am 
aware, in the biological or geological evidence at present acces- 
sible to render untenable the hypothesis that an area of the 
mid- Atlantic, or of the Pacific sea-bed, as big as Europe, should 
have been upheaved as high as Mont Blanc, and have subsided 
again any time since the Palaeozoic epoch, if there were any 
grounds for entertaining it.’* 
It is so obvious that the causes which lead to elevation and 
subsidence must react one upon the other, that I am tempted 
to speculate upon them and their effects on deep-sea basins. I 
have long been struck with the almost universal tendency to 
depression exhibited in areas occupied by deltas and estuaries. 
The thought has occurred to many, and has perhaps been most 
clearly expressed by Dr. Charles Picketts, that this subsidence 
is produced by the accumulation of sediment, f The cause 
appears insignificant, yet something must determine the move- 
ment of the Earth’s crust, and even an accumulation of a few 
feet of clay over several square miles may create disturbance, 
and eventually lead to a downward tendency. Supposing a 
sediment, 50 feet in depth and entirely submerged, to have 
displaced an equivalent of sea-water, we should have an in- 
creased pressure per square yard, taking the mean density of 
the materials composing a delta at 120 lbs. per cubic foot, of 
I rather more than 25,000 lbs., or about 34,848,000 tons per 
square mile. As soon as the whole of the sea- water on an 
area is displaced and movement has set in, every cubic yard of 
sediment deposited adds a weight of about 3240 lbs. ; and 
when we see that deltas have accumulated to depths of perhaps 
even beyond 1000 feet, and extend, as in the Mississippi, to 
19,450+ square miles, we can realize how vast a force is 
present. 
? * Review of the first volume of the publications of the Challenger. 
A at lire, vol. xxiii. p. 1. 
t Geol. Mag. 1872, vol. ix. p. 119. 
I X Report, on Mississippi U. 8. War Department , 1864, p. 434. 
Records of borings in deltas are, the Po, 500 feet, Ganges, 481 , Mis- 
