1883.] Geological Effects of a Varying Rotation of the Earth. 25 
tion of elevation, and that heavy deposits of limestone, on the 
other hand, mark a continued submergence, we may note nine 
great vibrations, to say nothing of several minor ones. We may 
enumerate the periods of depression, as the Huronian, Trenton, 
Niagara, Lower Helderberg (?), Corniferous, Sub-Carboniferous, 
Permian (?), Cretaceous, Later Eocene, and the Champlain already 
mentioned? When we remember that these formations have 
been studied almost exclusively in the higher latitudes, and 
that we have seen reason, from later epochs, to believe motions 
of opposite phase, in lower latitudes, we may find it, as far as we 
now know, strong corroboration in our theory. 
Before leaving this point, the writer would say, that after elab- 
rating the theory as given above, he was pleased to find an 
almost identical view expressed by Dr. Dawson,’ as follows: 
“We have seen, in the progress of our inquiries, that the move- 
ments of the continents seem to have occurred with accelerated 
rapidity in the more modern periods. We have also seen that 
these movements might depend on the slow contraction of the 
earth’s crust, due to cooling, but that the effects of this contraction 
might manifest themselves only at intervals. We have further 
seen that the gradual retardation of the rotation of the earth fur- 
-nishes a cause capable of producing elevation and subsidence of 
the land, and that this also must be manifested at longer or 
shorter intervals, according to the strength and resisting power 
of the crust. Under the influence of this retardation, so long as 
the crust of the earth does not give way, the waters would be 
driven toward the poles, and the northern land would be sub- 
merged, but as soon as the tension became so great as to rupture 
the solid shell, the equatorial regions would collapse, and the 
northern land would be again raised.” This corroborating view, 
from so experienced a geologist, guarantees that the ideas pre- 
sented above are not wholly visionary. 
III. We pass on to indicate briefly certain important lines of 
investigation in connection with our subject. 
(1.) A re-examination, from a mathematical and physical stand- 
point, of the possibility of such contraction of the earth, and 
_ such variation of its ellipticity, as this theory requires. Sir Wil- 
1 Compare Dawson, Story of Earth and Man, p. 178; Shaler, chapter on Ancient 
Glacial Periods, in his recent work on Glaciers; also, Dana’s Manual. ; 
2Story of Earth and Man, p. 291. 
