Oil Ci/ch's of Si'd'niiriifiiiion. — AS iHiHinx. •'>lt> 
|)(»ii!ttk'ss lu'iv, is oiu' cxphiiuitiou of tlu- periodicity of disturli- 
unces ill land areiis. There aiv otliers. Dr. Croll has shown liow 
the precession of the ecpiiuoxes combined Avith variations in the 
eccentricity of the earth's orbit must give rise to a secidar recur- 
rence of periods of refrigeration. When winter in one hemisphere 
occurs in aphelion and the eccentricity is a maximum, the pohir 
regions of that heniispheie will l)e covered with a cap of ice. and a 
climate of arctic severity will prevail. Meanwhile the other hem- 
isphere, beyond the e(iu:\tor. will have :i torrid climate. This 
state of affairs will continue nntil the i)recession of the eiininoxes 
.shall reverse the conditions. Then the other hemisphere will 
sutt'er refrigeration and a tropical climate Avill dispel the ice-cap 
from the first. Dr. Croll shows how these changes would cause a 
shifting of the etirths centre of gravity toward the jjole which 
sustained the ice-cap. and thus the (jceanic waters would rise on 
that hemisphere and deluge the land. Thus each hemisphere 
would undergo alternate phases of submergence coincident 
with the alternate periods of icfrigeration. This oceanic 
oscillation, Dr. Croll argues, accounts for the alternate beds of 
coal and shale so characteristic of the Cotd 3Ieasui-es in nil parts 
of the world. ((Minuite and Time. ) Here then we have another 
clear cass of [)eriodicity, T would add in this connection that 
movements of the internal fluid aiisiug from changes in the cen- 
ter of gravity, would, by virtue of the superior monu'utum of 
the central lieavier masses inaugurate a motion which would churn 
the lighter peripheral fluid into the mountain arches and crevices, 
in some cases simply i»roducing elevation, in others, giving rise 
to igneous eruptions upon the ci'ust. Tf the interior of the eartii 
liad the rigidity of steel, a shifting of the centre of giavity would 
nevertheless lower tiu' peripheral pressure on the far hemisphere, 
perhaps thus ])roducing li(piefactiou. This mighl admit of lava 
outflows, and. in fad. evidences of such occuiM'ences :ii-e not 
wanting in pohir regions. .Moi'cover, such a shifting of the 
earth's centre of giavity. e\-eu if ever so slight, might pro|)agate 
spherical waves passing outward and importing terrific imjnilses to 
the crust. Such waves would liiiug into contact ditfen-nt cliem- 
ic;d elements separated before by their relative specific gnivities. 
and this would promote intense calorific effects which would react 
upon the crust. It is possible that the rise of sea level, due to 
the ice-caj). would depn'ss the sea-ltottom and thus elevate the 
