SEPT. 1901. DINOSAUR BEDS OF GR. RIVER VAL’y, CoLt.—RIGcs. 2 
] 
This fact, as well as the homogeneous nature of the clays, suggests 
that their deposition took place in comparatively still, if not deep 
water. In the sandstone ledges, however, the positions of the bones 
show that they have been carried about. Rarely more than the parts 
of a leg or a short series of vertebre have been found connected, and 
more often isolated bones are found. This, taken in connection with 
the varying coarse and fine-grained sands and the persistent cross- 
bedding, is conclusive evidence of deposition by water currents. In 
fact the abundance of fossils in certain pockets of this sandstone and 
the manner of their occurrence are very similar to those of the 
Metamynodon Beds of the White River Oligocene in South Dakota. 
The sandstone ledges are of course more massive, but in their relation 
to the clays above and below there is evidence of a similar set of con- 
ditions. The varying clays with the horizontal banding and frequent 
nodular ledges also correspond closely to the clays comprising the 
Oreodon Beds of the White River. 
Let us attempt to trace the history of the Jurassic formation as 
evidenced by the nature of the rocks, the stratigraphy and the occur- 
rence of fossils: Given an arm of the Jurassic sea, fed by rivers and 
open to the ebb and flow of tide waters. Under these conditions the 
sediments washed down by the river everywhere accumulated slowly, 
and alternating with them thin ledges of limestone and gypsum were 
laid down. Occasional strata of sand accumulated by the action of 
retarded currents about the estuaries of streams. Later, by some 
change in levels, the ingress of sea water was cut off, but the outlet 
still remained and so ensued the gradual change from salt to fresh 
water. Then followed a period of comparatively uninterrupted depo- 
sition in which the green shale was laid down under still water. Along 
with it were deposited near the mouths of streams the occasional 
homogeneous beds of green sand. As the basin filled up and its out- 
let deepened, the lake became shallower until its bed was invaded by 
the shifting channels of broad and shallow streams. Its sand-bars 
have formed the cross-bedded sandstone ledges which mark the 
transition from the lower to the upper clays. With the shallower 
waters came the great land and shore reptiles and about the estuaries 
of streams their remains were deposited abundantly. 
Again the lake waters invaded the region and the deposition of 
sand in this locality was cut off. The period following was one of 
greater changes and probably of slower deposition than that preceding 
the river period. The presence of fine reeds or sedges shows that the 
water was shallow, at least in places, and parts of skeletons found on 
irregular surfaces imbedded in these reedy clays, suggest mud-bars or 
