342 NATURAL SCIENCE. May, 



Mexican Gulf. Likely enough in portions of the Gulf in time past 

 there may have been outflows of lava on the sea-bed. Volcanic 

 intrusions in the form of sheets insinuated between the sedimentary 

 beds are another form in which it is highly probable these inland 

 igneous forces have developed themselves. If there be any justice 

 in these inductions there must be immense accumulations existing in 

 the Gulf of Mexico, for its area is not above one-third of that from 

 which the sediments have beeen derived. *° In addition we have the 

 great denudations from the mountainous regions of tropical America 

 and from Mexico. 



The land sediments and the Hme and silica ehminated from the 

 waters of the sea by organic agencies, and ever being renewed by the 

 decomposition of the rocks through the solvent action of rain-water 

 aided by humic acids, must, together with igneous flows, intrusions 

 and ashes, be building up rock-groups which will eventually form the 

 framework of new land. None the less is this the case when the earth's 

 crust has been long in apparent repose ; for there surely will come a 

 critical time when they will begin to react on the earth's interior, so 

 that before being elevated into new land these portions of the earth 

 may go through the volcanic cycle of change. We might in this way 

 travel over the earth's surface and find in every part modified phases 

 of the same actions in progress — for our planet has not yet lost its 

 vitahty. All along the west coast of the two Americas volcanic 

 activity is adding to the thickness of the sea-bottom, and the ceaseless 

 denudation of the great mountain ranges provides a covering on a 

 still grander scale. On the Atlantic coast of South America the 

 conditions of the coast are more general, but here a still greater load 

 of sediment is being deposited by the Amazons, La Plata, Orinoco 

 and other great rivers. On the African coast it is the same, and two of 

 the greatest rivers of the world — the Amazons and Congo — pour their 

 tropical floods and the spoils of the land on opposite sides of the 

 Atlantic in the same parallels of latitude. 



I have shown that the sediment from 21,000,000 square miles of 

 land, the estimated area of the land draining into the Atlantic, would, 

 at the rate of one foot per 3,000 years, fill up the North and South 

 Atlantic, estimated at 21,000,000 square miles, two miles deep, in 

 32,000,000 years. ^' 



With all this variety I have little doubt that the deposits as a 

 whole will be differentiated from any that have gone before so as to 

 justly enable the time in which they were laid down to be called 

 a Period. 



'^ In a valuable paper, entitled, " The Gulf of Mexico as a Measure of Isostacy " 

 (Ann. Joxirn. of Science, vol. xliv., 1892, p. 188), McGee estimates the area of degra- 

 dation at 1,800,000 square miles, and that of deposition at 100,000 square miles or 

 one eighteenth. This may be true in relation to what is now taking place, but my 

 assumption is that the area of deposition has shifted from one locus to another, 

 being conditioned by elevation and subsidence. 



21 " Denudation of the Two Americas," Presidental Address, L'pool. Geol. 

 Soc, 1885. 



