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THE SCIENTIFIC MONTHLY. 
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and striations would present the same confused appearance spoken of before| 
The theory most satisfactory, and the one which has been universally accepted, is 
that known as “ the glacier theory.” This theory supposes that a glacier, confl 
nental in extent, once covered the northern portion of our continent; that it formed 
in the higher latitudes and moved towards the tropics; that it took up and tran®l r f|.«rvpf & 
ported in its southern progress large blocks of granite severed from primiti^Hj;." 
cliffs and hills, and mountains of the north; and that on melting these blocks | f 
rocks were deposited. Granting the existence of such conditions, the scratches = 
and grooves would be parallel—at least all produced by a given continent& if ^ jr & ^ 1 
glacier would be parallel. Granting the glacier, this is all very plausible ; but j ? | g .'* 2? 3 ^ 
whence came the glacier ? 
It was originally thought that the conditions requisite for the formation of Jo 
such continental glaciers as those of which we have spoken, was brought ahoi a p:: pOYag 00£gv? 
by a northern elevation; but the idea that this was the sole or principal cause 5 
continental glaciers was never accredited by many geologists, and was long sinflH^ o-g ~ 2? 3 5 
abandoned entirely. After the overthrow of this theory, it was suggested that^he *3 ? ? SS r* J 
requisite conditions might be produced by secular changes in the obliquity of the 
earth’s ecliptic. This theory received the support of the most advanced thinkers 
of the day, for a season, but eventually went the way of its predecessors. It is 
now maintained that the requisite conditions are produced by the variations of the 
eccentricity of the earth’s orbit. When the northern hemisphere has its winter 
solstice in aphelion, during a cycle of extreme eccentricity, the sun will be 
14 , 212,700 miles farther away than when the winter solstice of the same hemis¬ 
pheres occurred in perihelion during a cycle of extreme eccentricity. Conse¬ 
quently, when this hemisphere has its winter in perihelion during the time 
of maximum eccentricity, the amount of glaciation must be far less than when 
it has its winter in aphelion during the same maximum •eccentricity. 
The investigations of Herschel, Humboldt, Arago, Lyell, and others came 
near overthrowing this theory by demonstrating that neither increase ordimnution 
of the earth’s eccentricity, could influence to any great extent the amount of heat 
received by the two hemispheres,. But our author has redeemed the theory by 
showing satisfactorily that a considerable increase of the earth’s eccentricity 
beyond that which it has at present, would so modify the currents of the ocean 
as to greatly increase the amount of snow and ice in the northern hemisphere, and 
diminish proportionately the amount in the southern hemisphere. Not only this, 
but he has also established the periodicity of the recurrence of a glacial epoch. His 
work is a contribution of permanent value to geological science. 
Principia, or Basis of Social Science. —By R. G. Wright, second edition, 
Lippincott & Co., Philadelphia, 1876 . 
This is the unsuccessful attempt of a thinker unquestionably of ability to 
construct a system of social science which shall embody fundamental principles of 
universal application, a thing as visionary as Plato’s “ Republic,” as improbable as 
Swift’s “ Lillypute,” and as impossible as Mare’s “Utopia.” 
Missouri Botanical Garden 
George Engelmamh Papers 
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