PROCEEDINGS. 
535 
tion to a slight extent), acting upon the earth to produce internal tidal 
friction, are converted into heat, and thus we have a steady development 
of heat in the interior of the earth to supply what is lost by radiation. 
I do not suppose this heat sufficient to melt the rocks, nor do we in 
fact need to assume any high temperature in the interior of the earth. 
We have at present all the fluidity we need in the central portion of our 
planet. In the lower portion of our outer crust any breaks in the rocks 
are quickly healed by a welding process; it is in the quasi-viscous outer 
portion of the crust where pressures are less and rigidity slightly greater, 
and where innumerable faults break up the solid strata into fragments 
that slide on each other like the molecules of the small masses that we 
experiment on in our laboratories, that a steady tidal action produces 
the strains that continually cause new faults and crumples with their 
accompanying earthquakes, and is therefore most efficient in producing 
sensible heat. 
Our observed earth temperatures show an increase of one degree Fah¬ 
renheit for fifty feet of descent, but it is a very violent hypothesis that 
assumes this increase to continue at this rate for many miles in depth. 
The hardest materials that we know of would become plastic at ordinary 
surface temperatures and under the pressures prevailing at a depth of fifty 
miles; but with a slowly increasing temperature, they would become very 
plastic at a temperature of 1,000° Fahrenheit and the pressure prevail¬ 
ing at a less depth, say of twenty miles; so that the latter is the greatest 
thickness we need assume for our so-called solid crust. 
Now, it is only in this crust that by means of crushing earthquakes^ 
tidal friction, viscous tides, faulting, chemical action, and perhaps other 
causes, there takes place a continued evolution of internal heat. This 
heat has been conducted inward for ages, until the central sphere has 
attained a nearly uniform temperature; it has also been conducted out¬ 
wardly for ages to the radiating surface of our globe. As the evolution of 
heat still continues, we have on the continents attainel a pretty stable 
rate of temperature decrease of 100 degrees to the mile (and rather less, 
I suppose, under the oceans), but I do not suppose that at any time the 
hottest stratum within the earth has had an average temperature so high 
as a thousand degrees Fahrenheit (except in lava pockets). 
Of course I accept the view that the specially high temperatures attend¬ 
ing the formation of lava have been due to the local chemical action be¬ 
tween warm rocks and water penetrating to them under great pressure 
through crevices in the crust. 
The study of Geological Climate during and since the formation of 
Azoic metamorphic strata has led me to adopt the conclusion that surface 
geology, like volcanic, does not demand excessive temperatures ; it seems 
to me most reasonable to assume that the surface was never much warmer 
than 250 degrees Fahrenheit, but to allow that this temperature may have 
prevailed at the close of the Archseic epoch. 
At this temperature all the water of the ocean would exist only as vapor 
