Microscopical Light i?i Geological Darkness. — Clay pole. 225 
and dissolving action of water. Over and over again have 
these strata been broken up and swept away by rains and 
rivers, until the ancient crystallines have gradually been 
buried under their own ruins and now occupy comparatively 
a small part of the surface. None the less has every particle 
of the sedimentary strata, -except carbon and carbon-dioxide, 
been derived from their steady destruction, the amount of 
which must, of course, be approximately equal to the mass 
built up from their ruins. 
Had the geologist senses sufficiently exalted he might hear 
the miniature explosions, as one after another, or, many at 
the same instant , these little "sealed flasks" burst and dis- 
charge their highly compressed contents. In grinding down 
a thin slice, myriads of them are opened and their gases lost. 
So in nature, as erosion thins down their crystal walls, these 
ultimately become so weak that they can no longer with- 
stand the bursting pressure within and a microscopical ex- 
plosion ensues. 
The area of the dry land of the globe equals about 60,000,- 
000 square miles, and by far the greater part of it is covered 
with thick sheets of sediment. Deduction must be made for 
the areas where these are absent and the old crystalline rocks 
still form the surface. But, on the other hand, a large addi- 
tion is due on account of the sea-margins which for 200 or 
300 miles from shore are covered w'ith the wash from the land. 
With the deep sea I will not here deal. 
I assume, then, that one of these corrections will counter- 
vail the other and that the area of the sedimentary strata is 
equal to that of the present dry land or 60,000,000 square 
miles. Now the thickness of these rocks is very various, 
ranging from ten miles down to nothing. The former figure 
is seldom found, but it appears to me that to assume an aver- 
age depth of one mile is not unreasonable. This will give us 
for the whole mass eroded from the ancient crystalline rocks 
the sum of 60,000,000 of cubic miles. 
Regarding the quantity of carbon-dioxide contained by the 
rocks on which Prof. Tilden experimented, the following 
figures are taken from the report of his paper given in the 
Chemical News for April 9, 1897: 
