268 LAE fOOCRINAE NOR GEOLOGY. 
use it simply as a vague term signifying a relatively long lapse 
of time. I do not think we have quite unequivocal evidence to 
show that any one of these epochs was longer or shorter than 
another. For aught that we can tell all may have been of equal 
duration. One is apt to infer that the enormous erosion and 
accumulation evidenced by the Saxonian implies a much longer 
time than the corresponding work accomplished, say, in the 
Lower Turbarian epoch. But we may quite well be mistaken in 
our inference. A horse will do much more work in an hour than 
a man can accomplish in the same time. So the vast ice-sheets 
and glaciers of the Saxonian stage, in the work they were able 
to perform, must necessarily have surpassed the comparatively 
insignificant ice-flows of a later date. But it is obvious that the 
erosion of a rock-basin 600 feet deep and twenty-five miles long 
need not have occupied more time than was required for the 
excavation of a basin hardly a mile in length and only a few 
yards in depth. Again, the Saxonian and Polandian bowlder- 
clays and fluvio-glacial gravels far surpass in importance those 
of the Mecklenburgian stage, but he would be a rash geologist 
who should maintain that the thicker and more extensive accumu- 
lations necessarily took the longest time for their formation. I 
am not aware of any evidence that would compel me to infer 
that the small local glaciers of the Lower Turbarian did not exist 
for as long a space of time as their gigantic predecessors of 
earlier glacial epochs. Once more, if we take into consideration 
the many complex climatic and geographical changes and the 
several migrations of flora and fauna which characterized the 
later phases of the Glacial Period, we shall hesitate before con- 
cluding that the Mecklenburgian and succeeding stages are less 
entitled to be classed as epochs than the more strongly contrasted 
stages that preceded them. And similar remarks apply to the 
interglacial stages. I do not know how we are to estimate 
the relative duration of each particular genial epoch. It can be 
shown that great valley-erosion took place during the earlier 
interglacial stages; and the persistence of these epochs is still 
further indicated by the depths to which surface-weathering 
