302 CONTRIBUTIONS TO ECONOMIC GEOLOGY, 1902. [bull. 213^ 
assignment to Cretaceous or Tertiary — also carries bitumens. The 
deposit occurs in the northern portion of Middle Park, almost in the 
heart of the Rocky Mountains. It is here an asphalt resembling gil 
sonite and occupying an irregular fissure or series of fissures in the 
clays, sandstones, and conglomerates of the formation referred to. 
The occurrence is, so far as known, limited to the immediate region 
in which it is found, and is thus comparatively isolated, the nearest! 
asphalt being the gilsonite found along the Colorado-Utah line, 150ji 
miles distant. 
The bitumens of the Tertiary horizons are apparently confined to 
the West — to Colorado, Utah, and California. The various divisions 
of the Eocene in eastern Utah and just across the line in Colorado are 
noted both for the variety of their asphalts and for the size of their 
veins. It is in this region, of the White and Green rivers and the 
celebrated Book Cliffs, that the great veins of uintaite are found andj 
that the minor seams of wurtzilite, ozocerite, and nigrite occur. 
Bituminous limestones are also of wide distribution, though confined 
to the Green River shales. The asphaltites occur in fissures both in 
this formation and in those overlying, especially the Bridger and 
Uinta. Maltha springs also occur, and even petroleum is reported 
in one of the members of the Cretaceous, a few miles east of the 
Utah-Colorado line. The Green River shales are noted throughout 
the West for their bitumen contents, and it is surmised that they are 
the source of the asphalts, at least, of this vast area. The variation 
in the ultimate material as it to-day fills one fissure or another is per- 
haps due in part to a change somewhat allied to fractional distillation 
in petroleum technology and in part t<> the degree to which oxygen 
absorption lias been carried on. In any event, the variety of bitumen 
found can hardly excite wonder when considered as to the origin of 
the material and the differentiations that take place in artificial dis- j 
filiation. It is, indeed, to be expected. 
The occurrence of bitumens in the Neocene is confined to California. 
The rocks of this period here embrace a heavy series of shales with 
local sandstones, tuffs, etc. — the Monterey formation; a conspicuous 
body of massive sandstone with a minor proportion of shales, known 
as the San Pablo formation, but in doubt as to its position as a mem- 
ber of the Miocene or Pliocene, and hence in the region of its occur- 
rence regarded as Middle Neocene; and a succession of sandstones, 
conglomerates, and clays, probably Pliocene, but for the present 
termed Upper Neocene, with the specific name Paso Robles assigned 
to it. 
In addition to the foregoing, there are certain and important sand- 
stones and sand aggregates of somewhat doubtful age, but where 
encountered seeming to lie beneath the shales of the Monterey rather 
than in them, and often against and upon granite. Such is their 
occurrence in the vicinity of Santa Cruz, and again at one or two points 
