80 ANIMAL LIFE AS A GEOLOGICAL AGENCY. 
once abundant on the North-Western prairies, have been so 
nearly extirpated by the inroads of half-wild vegetables which 
have come in the train of the Eastern immigrant, that there is 
reason to fear that, in a few years, his herbarium will constitute 
the only evidence of their former existence.* 
There are plants—themselves perhaps sometimes stragglers 
from their proper habitat—which are found only in small num- 
bers and in few localities. These are eagerly sought by the 
botanist, and some such species are believed to be on the very 
verge of extinction, from the zeal of collectors. 
Animal Life as a Geological and Geographical Agency. 
The quantitative value of animated life, as a geological 
agency, seems to be inversely as the volume of the individual 
organism ; for nature supplies by numbers what is wanting in 
the bulk of the animal out of whose remains or structures she 
forms strata covering whole provinces, and builds up from the 
depths of the sea large islands, if not continents. There are, it 
is true, near the mouths of the great Siberian rivers which 
empty themselves into the Polar Sea, drift islands composed, in 
an incredibly large proportion, of the bones and tusks of 
elephants, mastodons, and other huge pachyderms, and many 
extensive caves in various parts of the world are half filled with 
the skeletons of quadrupeds, sometimes lying loose in the earth, 
sometimes cemented together into an osseous breccia by a cal- 
careous deposit or other binding material. These remains of 
large animals, though found in comparatively late formations, 
generally belong to extinct species, and their modern congeners 
or representatives do not exist in sufficient numbers to be of 
sensible importance in geology or in geography by the mere 
mass of their skeletons.+| But the vegetable products found 
* Report of Commissioner of Agriculture of the United States for 1870. 
+ Could the bones and other relics of the domestic quadrupeds destroyed by 
disease or slaughtered for human use in civilized countries be collected into 
large deposits, as obscure causes have gathered together those of extinct 
