128 THREE CRUISES OF THE “ BLAKE." 
We may go back to the time when all the water of our oceans, 
rivers, and lakes existed only as vapor suspended round the 
heated earth, — forming, as has been said by Mallet, an atmos- 
phere of steam, producing greater extremes of light and heat 
than are known to-day, enormous differences between summer 
and winter, and very striking contrasts between the polar and 
equatorial regions. When water was first deposited, on the 
cooling of the crust, ice may have existed at the poles, while the 
equatorial seas may still have been too hot to sustain life. Such 
contrasts in temperature must have occasioned oceanic currents 
unparalleled in the present state of the globe. Consequently 
the disintegration of the rocks forming the crust of the globe, 
subjected as they were to the torrential rains, the powerful cur- 
rents, and the extreme solvent powers of water at the high tem- 
peratures then existing, must have been far greater and more 
rapid than now. It'is not surprising, therefore, if we find the 
earlier stratified rocks deposited through agencies similar to 
those now acting, and yet formed under conditions having no 
exact parallel in our times. 
The material brought down to the sea from the basin of the 
Mississippi has been accurately measured by Humphreys and 
Abbott, and their report states that it would require five millions 
of years, at the present rate of denudation, to carry off a thick- 
ness of one hundred feet. As far as we can judge from the 
deposits off the mouth of the Mississippi, the mud brought down, 
while it has made an extensive submarine deposit, projecting far 
beyond the general continental line, has not been detected be- 
yond a distance: of about one hundred miles from the passes. 
To this must be added the matter held in solution by rain-water, 
the amount of which depends principally upon the nature of the 
rocks forming a hydrographic basin. Frankland has calculated 
that for every 100,000 tons of water there were 39 tons of car- 
bonate of lime and magnesia, and 1,018 tons of sulphates; so 
that, according to Reade, it would take twenty-five millions 
of years to accumulate the sulphates now in the sea, and only 
480,000 years to renew the carbonates ; but that it would take 
about two hundred millions of years to replace the existing sup- 
ply of chlorides. The carbonates have, of course, been con- 
