44 
THE (iKANl) GASTON DISTRICT. 
liorizon, and in some otliers its tliickness becomes more than 1 00 feet. But 
on the whole it is a remarkably persistent bed, and its persistence is all the 
more striking when we consider the coarseness of its texture; for no beds 
are so variable as the coarse ones. This member has been named by Pow- 
ell the Shin-4-rump Conglomerate. The name Shimirump he also applied 
provisionally to a large group of beds in which the conglomerate is included. 
For several years it was thought very probable that these beds were a part 
of the Triassic system, though no positive proof could be cited to sustain 
that presumption. In the summer of 1879 Mr. C. D. Walcott, of this survey, 
at length found some limestone bands near the base of Powell’s Shindrump, 
which seem to establish pretty conclusively their Permian age. But the 
fossils so far discovered have only a small vertical range, and lie near the 
base of the group. Above them are many hundred feet of beds which yield 
no fossils at all. While some of them are unquestionably Permian, it still 
remains to find the horizon where the Permian ends and the Trias begins. 
The Trias is as destitute of fossils as the Permian, excepting, however, some 
which are useless for determining age. In cases like this the geologist finds 
himself in trouble. He is quite sure that he has beds of two distinct ages; 
and he must, for purposes of discussion, separate them somehow; if not by 
a natural and unmistakable dividing horizon, then by an arbitral-}^ and pro- 
visional one, subject to amendment by future research. As we examine 
the Triassic series from the middle downwards the various beds are so much 
alike in general character that a divisional horizon seems impossible. And 
if we examine the Permian upwards the same similarity is observed. The 
only member which forms a sharp contrast is the Shinarump conglomerate; 
and this stratum was selected by Mr. Walcott for the plane of the division. 
Much more to the purpose is the fact that the conglomerate rests uncon- 
formably upon the Permian shales below. The unconformity, however, 
is by erosion only without any discrepanc)" of dip. The shales were slighth’ 
eroded before the conglomerate was laid down, but neither in the emergence 
nor in the following submergence was the rigorous horizontality of the beds 
at all disturbed. Adopting the plane of unconformity rather than the stratum 
itself as the dividing horizon, the conglomerate obviously becomes the 
basal member of the Trias. 
