60 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
ming. The problem selected was of such a nature that its study 
could be pursued profitably in both regions. The results of the in- 
vestigation are embodied in the following pages. In the preparation 
of the paper I have been greatly aided by the use of specimens, maps, 
and other data, kindly placed at my disposal by Professor Jaggar. 
THE PROBLEM OUTLINED. 
In both districts, high above the present water courses, occur exten- 
sive deposits of gravels that are apparently of fluviatile origin. In 
some cases their distribution suggests that they have been laid by the 
streams of today, before the latter cut to their present depths; but in 
other cases the gravels are so disposed that they bear little relation to 
the channels of the water-ways now found in the region and show that 
considerable changes of drainage have taken place since they were 
deposited. The slopes of many of the valley sides indicate more than 
one period of incision. The upper slopes are fairly gentle and seem 
to be related to the high level gravel deposits and to certain abandoned 
saddles in the divides, which appear to be of stream origin. The 
lower slopes are steep, often precipitous, and descend to the beds of 
the present creeks. ‘There are numerous sharp bends in the courses 
of the streams of today, together with abandoned gateways and chan- 
nels. There is, moreover, a significant defiance of structure in some 
places, while near by a high degree of adjustment has been attained. 
The problem may, then, be stated interrogatively thus: What was 
the character and direction of the drainage that deposited the gravels ? 
What changes have since taken place? How and in what order did 
they occur? 
METHODS or Work. 
For the solution of this problem the writer has had recourse to both 
field and laboratory methods. In the field the gravels were mapped; 
their altitudes were determined at numerous places by means of an 
aneroid barometer, and specimens of the pebbles were collected at 
representative localities. The slopes of the valley sides were studied 
and their relations shown by sketches. The elbows of capture were 
visited and an attempt was made to work out their history. 
In the laboratory the specimens from the Black Hills, numbering 
