412 Notices of Memoirs—Professor J. W. Gregory— 
of the contemporary organisms had chitinous shells or were soft- 
bodied. Paleontology begins with the appearance of hard-bodied 
organisms ; it can only reveal to us the dawn of skeletons, not the 
dawn of life. We are dependent for knowledge of the climate and 
geography of Hozoic time to the evidence of the sediments, of which 
there are great thicknesses beneath the fossiliferous rocks in most parts 
of the world.! 
II. 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 endowed with names; and the 
manifold activity of water and air in moulding the surface is duly 
appreciated, it is not surprising to find that the centre 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 recognised 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 depositing 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 utilisation of soils and waters, and 
for the pursuit of deeper deposits of ore. 
If a shaft be sunk at any point on the earth’s surface a Ponape 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 he beneath them. But already there are indications that 
the mystery of the ‘ beyond’ is not so impenetrable as it seemed. 
1. Zhe Nebular and Meteoritie Hypotheses.—The eighteenth century 
explained the history of the earth by the nebular hypothesis of 
Laplace. Geologists respectfully adopted this idea from the 
astronomers; they accepted it as one of those essential facts of the 
universe with which geological philosophy must harmonise. The 
resulting theory represented the earth as originally a glowing cloud 
of incandescent 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 
condensing vapour, forming oceans which became habitable as the 
temperature further fell. The whole earth was thought to have had 
_| Such are the Algonkian sediments represented by the Huronian and Algonkians 
ot America, the Ale onkians of Scandinavia, the Karelian of Finland, the Briovarian 
ot North- West France, he Heatheotian of Australia, the Transvaal and. Swaziland 
systems of South Africa, the Dharwar and Bijawar ‘systems of India, the Itacolumnite 
series of Brazil, etc. 
