1877.] The Age of this Earth. 289 
and accept the tidal action as a part of the dynamical action, 
that helps to keep the whole sphere in equilibrium. 
We now come to a mixture of mathematics and geology. 
The late Sir Charles Lyell, calculating from the present growth 
of the dry land, put the age of this earth at 300,000,000 years, 
the fossiliferous strata at 240,000,000, and the Glacial epoch at 
1,000,000 years ago. 
The earnest and accurate mathematician, Croll, endeavors to 
modify the geological epochs by a calculation “ at once benefi- 
cial, simple, and complete. . . . The land has been many times 
under the sea, and the most violent changes of climate have suc- 
ceeded one another.” Causes are produced to show “ that every 
ten thousand years, or thereabouts, the winter of the northern 
hemisphere will occur, . . . whenever any records are left of the 
Glacial epoch, a general subsidence of the land followed closely 
on the appearance of the ice, . .. the extent of submergence 
will be in proportion to the weight of ice, . . . glaciation would 
be transferred from one hemisphere to another every ten thou- 
sand years. There would be elevation of the land during the 
warm period, and subsidence during the cold.” 
The article goes on to tell us that “ the discovery of Mr. Croll 
upsets the whole scale of geological time.” Croll gives four 
periods of glaciation, the last of which was 200,000 years ago; 
so that the Glacial period is reduced to that age, the date of 
the fossiliferous rocks to 48,000,000, and the age of the earth to 
60,000,000 of years. 
_All glacial epochs are supposed to have left their marks be- 
hind them. Croll says they oceur every ten thousand years, but 
the writer of the article has left out twenty of these periods, and 
Selected one of Mr. Croll’s without knowing whether Sir Charles 
Lyell referred to that particular period or to another which 
Croll tells us took place 850,000 years ago. As figures go in 
this matter, Sir Charles would have a very small margin of 
150,000 years if he worked on the traces of this period. The 
Writer of the article seems to have had an object in his figures 
by reducing them as nearly as possible to his own. 
€ must now consider two more facts brought forward by 
Croll in support of his calculations : — 
First : « The land has been many times under the sea.” . . 
‘ The submergence will be ‘in proportion to the weight of ice. 
he latter quotation cannot be proved ; ice vanishes, and its 
pa cannot be known. Submergence of land takes place all 
5. 
OL. XI — xo, 19 
. 
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