1896. ] NEW YORK ACADEMY OF SCIENCES. 259 
over the erosion; and the bottom of Lodore, which formed the 
only outlet to the park, rose many feet above the valley, bank- 
ing up the waters of the river into a lake. 
Slowly this lake increased in size and stretched out over the 
valley of Brown’s Park until the gradual cessation of those oro- 
genic movements that elevated the Uintah range again threw 
the balance on the side of erosion, causing the lake as slowly to 
subside and disappear. It may at first seem strange that a sim- 
ilar damming of the river did not take place at Horseshoe Cafion 
where the Green River enters the fold; but an explanation of 
this lies in the relative position of the two cafions. That by 
which the river enters the park lies on the edge of the fold; its 
exit, on the other hand, cuts directly through the axis of the 
mountain. It is evident ata glance that the point where the 
river first began to cut into the quartzite was in this axis, z. ¢., 
in the point of earliest elevation. Therefore the erosion in the 
long cafion on the edge of the fold would have had only the 
uppermost and least crystalline strata to deal with when Lodore 
had already penetrated far within the hard, metamorphosed core 
of the formation. 
It is here that the importance of our observations concerning 
the dip of the beds in the valley becomes apparent. The per- 
fect horizontality of the Brown’s Park beds throughout their 
entire extent shows that it was not until the elevation had 
ceased that Lodore was again cut down to its old level and the 
waters of the lake allowed to escape. 
The sharp dip to the south of the exposures of Green River 
Shales on the northern edge of the park, on the other hand, 
shows that they were involved in the fold and that the elevation 
must have continued long after the Green River Lake had become 
a thing of the past. 
CoLumBIA CoLLecE, May, 1896. 
EXPLANATION OF PLATE. 
Plate XVIII. Geological Maps of the Region about Brown’s Park, Utah. 
The geography is based on the map by C. A. White, in the Ninth Annual 
Report of the Director of the U. S. Geological Survey. 
