452 RECENT ADVANCES IN GEOLOGY. 
was a marked modification of forms. Dr. Leidy, in his late 
work on the extinct mammalian faune of Dakota and Ne- 
braska, states that, of the thirty-two genera of Miocene 
animals, not one occurs in the Quaternary formation of 
North Ameriea. In comparing the Miocene and Pliocene 
faunæ with each other, as represented mainly by the remains 
from the Mauvaises Terres and the Niobrara River, scarcely 
a genus is common to both. “In view,” he continues, “of 
the consecutive order and close approximation of position of 
the two formations and fauns, such exclusiveness would 
hardly have been suspected." The greater similitude of the 
Miocene and Pliocene fauns with the contemporaneous 
faunæ of the Old World, has led him to suggest that the 
North American continent was peopled, during the Tertiary 
Epoch, from the West. ‘Perhaps this latter extension,” he 
continues, “occurred from a continent whose area now forms 
the bottom of the Pacific Ocean, and whose Tertiary faunæ 
is now represented east and west by the fossil remains of 
Ameriea.on the one hand, and of Asia, with its peninsula, 
Europe, on the other." 
The topographieal features of the two continents and the 
hydrographieal soundings of the two oceans, render this 
supposition probable. Between Ireland and Newfoundland 
there is a great plateau, which an elevation of the earth's 
crust to the extent of a few thousand feet would convert 
into dry land; and Behring's Straits, which now separate 
Asia and North America, are, at their narrowest points, but 
thirty miles wide, and their shallowest depth is but twenty- 
five fathoms. 
And here the paleontologist comes to the aid of the 
hydrographer, and, by their joint labors, the one renders 
probable what the other has conjectured as possible —the 
former union of the two hemispheres. Zoology would indi- 
cate that such was the fact during the Pliocene Epoch, in 
which will probably be found the origin of those mammalian 
types eontemporary with the elder man, and represented by 
