180 J. LeConte—Old River-beds of California. 
moth, the great mastodon and a tapir, undistinguishable from 
the living species, indicating a Quaternary rather than a Plio- 
cene age. These Quaternary remains have been in several 
instances found under the volcanic caps in the lowest blue- 
gravel, next to the bed-rock. Several examples of this kind 
are now in the museum of the University. Some human re- 
mains and implements are also supposed to have been found in 
this detritus, but the authenticity of these is disputed by many. 
n several cases I observed in the vertical cliff of detritus 
distinct curved lines of discontinuity, concave upward, indicat- 
ing sub-channels cut in the main mass of detritus. Undoubt- 
edly the main channel had been first filled, then partly swept 
out by erosion, and then re-filled. This observation is impor- 
tant, as it seems demonstrative of a true river agenc 
e lower portion of the detritus, the so-called blue gravel, 
differs from the upper portion partly in structure, but chiefly 
in color. In structure it is almost if not quite devoid of 
lamination ; and when the rock fragments are sub-angular it is 
almost undistinguishable from true é// or ground-moraine. In 
most cases, however, its pebbles and bowlders are perfectly 
rounded. Its blue color is undoubtedly due to the fact that its 
iron is in the form of ferrous instead of ferric oxide. There is 
no such line of demarkation between the blue and the red 
gravel as would indicate a different origin. On the contrary 
the irregularity of the plane of contact and the shading of the 
color shows a downward progressive oxidation of iron, greater 
in some places than in others. 
e capping.—Above the detritus which constitutes the 
main portion of the filling of most of the old river-beds, we 
nearly always find a capping of volcanic matter 50 to 150 feet 
thi This is sometimes aa basalt underlaid by tufaceous 
conglomerate, but more usually tufaceous conglomerate only. 
3. 
ll 
Fig. 3,—SS, slate bed rock; RR, old river bed; r, present river bed; GG, old 
river gravel; VV, volcanic conglomerate. 
In this latter case, however, the presence of scattered blocks of 
basalt on the surface often indicates the former existence bi a 
usual form. When the consists of tufaceous conglomer- 
ate above, the whole thic 
