L. Semann on Geological Phenomena in the Solar System. 39 
one of the gravest objections to the idea of a’ common origin of 
the moon and the earth is the apparent absence of water and air 
from the surface of our satellite, thus seriously embarrassing 
those geologists who attribute terrestrial volcanic phenomena to 
the intervention of these expansible elements, 
owever we admit for the earth and the moon an identi- 
eal and simultaneous point of departure we can understand that 
their cooling has taken place at a rate nearly proportioned to 
their volume. That of the moon being about two hundreths the 
volume of the earth, its temperature, if we admit an equal con- 
ductibility, will have decreased with a rapidity fifty times greater, 
so that the geological epochs of the moon will have been in the 
same proportion shorter than the corresponding epochs on the 
earth, up to the time when the solar heat began to be an ap: 
preciable element, The moon has then advanced much more 
rapidly than the earth in the series of phenomena through which 
must pass, and we may therefore logically suppose that 
our globe will one day offer the same general characters as are 
how presented by the moon. 
e believe then that the water which covers the surface of 
the earth and the air which surrounds it will one day disappear, 
aS @ necessary consequence of the complete cooling of the inte- 
tior of our planet. Rocks, with few exceptions, readily absorb 
moisture, and the more crystalline varieties are the most porous ; 
Wwe need not, however, consider the quantity of water which 
Tocks may imbibe in this way, for the total amount of this ele- 
ment on the earth’s surface is so small when compared with the 
Whole mass of the globe, that the ordinary processes of chemical 
analysis would not detect its presence. If we take the mean 
depth of the ocean at 600 meters* (=1968 feet), its weight will 
De equal to one twenty-four-thousandth of the earth, which be- 
lng reduced to decimals, would give for 100 parts, 
- - - - - 
eee ae 
Water, - - a a ne - ‘ pa as mm 0042. 
In the Bulletin of the Geol. Society of France, (2d series, vol. 
x, p. 131,) Durocher has published a series of experiments made 
into the structure of rocks, such as the fel micas, horn-. 
blende and pyroxene, and which are regarded as anhydrous - 
exposed to moist air, the proportion of water being determined 
both before and after; it wi a6 to 
give the amount of water found after exposure. The orthoclase’ 
Utoé absorbed in this way 0°41 for 100 parts, while the mean’ 
* This depth is deduced from the comparison of the relative areas of land and” 
Water which are Arcane 128, (he Aviation and depression of the surface being 
imed as proportional to the roots of their surfaces. (Saigey, Physique du 
Globe, 282.) The depth of the Bacifie Ocean as deduced by Bache from the earth- 
quake wave of Dec. 1854, was about 13,000 feet.—(This Jour. [2], xxx, 83.)--Kpe, 
