$22 RESUME AND THEORETICAL DISCUSSION. 
neous character. Not unfrequently, however, we find the most abrupt transi- 
tions from one kind of deposit to another, or sudden changes in the size of 
the component materials, indicating corresponding variations in the physical 
conditions at the time of the deposition of the mass. 
Perhaps it is more difficult to account for the remarkable uniformity which 
is displayed by the gravel deposits in certain localities, than it is to give 
satisfactory reasons for the ordinary variations in the size and nature of the 
component parts of the detrital masses. That in some places there should 
be a thickness of three or four hundred feet of gravel, almost homogeneous in 
character from top to bottom, seems to imply a persistent sameness in the 
work of the erosive agencies hardly to be expected. But it must be remem- 
bered that we usually see only a very small portion, longitudinally, of what 
once formed a continuous deposit resulting from erosion along one channel. 
It is only within very narrow limits that such homogeneous accumulations 
have taken place, and then only in very exceptional circumstances. 
In general, the variations in the mass of the gravel deposits are frequent 
and abrupt. There is no difficulty in understanding that these would fre- 
quently be sorted out by currents shifting in direction and varying in force, 
so as to present themselves in a succession of strata of irregular thickness, 
made up of fragments of very different sizes and very unequally water-worn. 
There is also the additional series of complications arising from the presence, 
during the later period of the gravel epoch at least, of voleanic materials in 
all kinds of forms, which help to swell the thickness of the formation. Vol- 
canic vents during the Tertiary epoch, as they do now, sent forth their 
products in a great variety of forms: lava, which, rendered fluid by heat, 
flowed down in the most accessible depressions of the surface, and then 
hardened to an almost indestructible rock; ashes, which may have been 
carried to a great distance from their place of origin either by the wind or 
by currents of water, which finally deposited them in the form of mud; frag- 
mentary and brecciated masses, where the lava has been ejected in a solid 
form, but broken into large blocks and not pulverized to ashes, —all these 
varieties are common, but very irregularly distributed throughout the gravel 
region. That portions of the solid eruptive rock should be abraded and 
become water-worn, so as to form a voleanic gravel, is what would naturally 
be expected. One principal reason why such instances are rather rare is, 
that the flows of solid lava belong to the later part of the gravel epoch, 
when the eroding agencies had already slackened in their work. 
