ldbidgb.] ASPHALT AND BITUMINOUS ROCK DEPOSITS. 305 
tones and its evident prevalence in the underlying Green River shales 
ndicate the latter series as the one-time source of the oil which on 
sntering the fissures was converted into the asphaltites. Moreover, 
ocally, for a foot or two adjacent to the veins, the sandstone is filled 
vith interstitial gilsonite, which is evidence of infiltration from the 
ietrolenm-filled fissure into the sandstones rather than into the fissure 
:rom the rock on either side. The channels by which the shales were 
Irained are cracks that extend from the bottom of the main portion of 
;he fissures, in some instances several hundred feet, into the underlying 
)itumen-bearing beds, but even here the draining of the strata must 
have been marvelously rapid to have been so complete as the condi- 
tions indicate before being interfered with by the closing of the fis- 
sures from the settling or readjustment of the shale, always a rock of 
exceeding instability. The writer believes, however, that the filling 
of the fissure could have been derived from no other source. The 
origin of the cracks is, of course, well understood. They occur in all 
formations and in all localities and are a concomitant feature of fold- 
ing, though perhaps at times developed from shrinkage 
In the filling of all reservoirs, whether fissures or sandstones, the 
investigator is struck Avith the almost inevitable slowness of the proc- 
ess and the vastness of the area of fine-grained sediments that must 
have been drained to yield the supply absorbed. Then, too, in the 
case of the asphaltites the hardening must have been very gradual, the 
material passing through a viscous stage, during which fragments 
dropped from the walls into the bitumen and yet were supported, even 
as a rock is supported upon the surface of a thickening maltha pool at 
the present day. After solidification was complete the crushing strains 
from readjustment of the strata became manifest in the penicillate 
structure developed in the asphalt next to the walls of the vein. It is 
probable that during the filling of the crack this readjustment was 
continuously going on, but it could become evident only after the vein 
material had been hardened sufficiently to record it. 
Bull. 213—03 20 
