320 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1907. 
Both these conceptions are now only of historic interest. The 
irregular individual shape of the earth is expressed by its description 
as a geoid. The processes which have produced its varying shape 
have also controlled its geological history and evolution, for they 
‘cause disturbances of the crust, which affect the whole earth simul- 
taneousiy; and so the geographical agents are given similar work 
and powers at the same time in different places. 
Hence there is a remarkable world-wide uniformity in the general 
characters of the sedimentary deposits of each of the geological 
systems. The last pre-Cambrian system includes thick masses of fel- 
spathic sandstones alike in the Torridonian of Scotland, the sparag- 
mite of Scandinavia, the Keweenawan sandstones of the United 
States, and perhaps also the quartzites of the Rand. The Cambrian 
has its graywackes and coarse slates and its numerous phosphatic 
limestones, the Ordovician its prevalent shales and slates; the Silu- 
rian its episodal limestones and shales. The Devonian has its wide 
areas of Old Red sandstones as a continental type, while its marine 
representatives show the prevalence of coarse grits and sandstones 
in the lower series, of limestones and slates in the middle series, and 
the recurrence of sandstones in the upper series; and this sequence 
occurs alike in Northwestern Europe, in America, and Australia. 
The Carboniferous contains the first regional beds of thick limestone 
and the first important Coal Measures. The Trias is as characterized 
by rocks indicating arid continental conditions in America and: Aus- 
tralia and South Africa, as Professor Watts has shown then pre- 
vailed in the neighborhood of Leicester. In the Mesozoic era we 
owe to Suess the demonstration of the world-wide influence of those 
marine encroachments or “ transgressions ” whereby the great conti- 
nents of the Trias were gradually submerged by the rising sea. 
Speaking generally, there is a remarkable lithological resemblance 
between contemporary formations in all parts of the world. This 
fact had been often remarked, but was usually dismissed as due to 
a number of local isolated coincidences of no special significance. 
But the coincidences are too numerous and too striking to be thus 
lightly dismissed. They are among the indications that the main 
earth changes have been due to world-wide causes, which led to the 
predominance of the same types of sedimentary rocks during the same 
period in many regions of the world. 
The conditions that govern the geological evolution and general 
geography of the earth are probably due to the interaction between 
the earth’s crust and the contracting interior; they may take place 
as slow changes in the form of the earth, causing the slow rising or 
lowering of the sea surface, or the slow uplift or depression of regions 
of the earth’s crust; or they may give rise to periods of violent vol- 
canic action in many parts of the earth, between which may be long 
