314 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1907. 
THE GEOLOGY OF THE INNER EARTH. 
Now that this geological survey of the earth is in rapid progress; 
while the history of life has been written at least in outline; the chief 
fossils, minerals, and rocks have been described and generously en- 
dowed with names; and the manifold activity of water and air in 
molding the surface is duly appreciated, it is not surprising to find 
that the center of geological interest is shifting to the deeper regions 
of the earth’s crust and to the problems of applied geology. The 
secrets of these deeper regions are both of scientific and economic 
interest. They are of scientific importance, for it is now generally 
recognized that the main plan of the earth’s geography and the 
essential characters of the successive geological systems are the 
result of internal movements. The relative importance of those 
restless external agents that we can watch, denuding here and de- 
positing there, has been exaggerated; probably they do little more 
than soften the outlines due to the silent heavings produced by the 
colossal energies of the inner earth. 
The study of the deeper layers of the crust is of economic interest, 
for, with keener competition between increasing populations and with 
the exhaustion of the most easily used resources of field and mine, 
there is growing need for the better utilization of soils and waters and 
for the pursuit of deeper deposits of ore. 
Tf a shaft be sunk at.any point on the earth’s surface a formation 
of Archean schists and gneisses would probably always be reached; 
and, working backward, geological methods always fail at last—in 
primeval, Archean darkness. The Archean rocks still hide from us 
the earlier period of the earth’s history, including that of all rocks 
which now lie beneath them. But already there are indications that 
the mystery of the “ beyond ” is not so impenetrable as it seemed. 
1. The nebular and meteoritic hypotheses.—The eighteenth century 
explained the history of the earth by the nebular hypothesis of La- 
place. Geologists respectfully adopted this idea from the astrono- 
mers; they accepted it as one of those essential facts of the universe 
with which geological philosophy must harmonize. The resulting 
theory represented the earth as originally a glowing cloud of incan- 
descent gas, which slowly cooled, until an irregular crust of rock 
formed around a gaseous or molten core; as the surface grew cooler, 
the depressions in the crust were filled with water from the condens- 
ing vapor, forming oceans which became habitable as the temperature 
further fell. The whole earth was thought to have had a long period 
with a universal tropical climate, under which coral reefs grew where 
flow our polar seas, and palms flourished on what are now the Arctic 
shores. Still further cooling had established our climatic zones; and 
it was predicted that in time the polar cold would creep outward, 
