74 The American Geologist. February, i904. 
the Niobrara. The Niobrara in this part of the Dakotas cer- 
tainly does not exceed a thickness of 60 to 100 feet, while in the 
area west of the 97° it has been reported to gradually and con- 
stantly gain in thickness (comp. U. S. G. S. Irrigation Paper 
No. 34, p. 16). In the southern part of the county its base has 
been observed at a general level of 1375 to 1380 feet above sea. 
The Tertiary Sands. 
The Tertiary deposits are to a great extent concealed by the 
overlapping drift. They seem to be represented only by a 
stratum of very fine sand, which is, however, not continuous, 
but is rather distributed in the form of lenses, which seem to 
thin out toward the periphery, thus indicating the drainage- 
system during Tertiary times and more especially during the 
Miocene and Pliocene. Wherever they may be revealed by 
deep borings or in natural or artificial exposures, it will be 
found somewhat difficult to distinguish them from the overly- 
ing sands and gravels of the drift. Where they are partly hid- 
den from view by the sHding of deposits higher up, their pres- 
ence is betrayed by the way in which they aid a stream to wid- 
en out its channel, transforming it into a broad vallev by yield- 
ing readily to the scouring action of the currents, while the 
overlying clays, deprived of their base, tumble, sometimes in 
bulky masses, into the rushing stream. This phenomenon, 
which has been observed by the writer along the Missouri at 
Dakota Cit}^ where in 25 years from a farm of 160 acres all 
has been devoured by the raging stream with the exception of 
7^ acres, seems to have also prevailed during the formation of 
the broad valleys of East and West Brule creeks, as well as the 
main stream and the wider portions of the valley of the Big 
Sioux. The data obtained by the writer are not sufficient to 
trace the outline of the drainage system mentioned above, which 
may become the subject of a more detailed study, when farm- 
ers will become more enterprising and undertake morQ exten- 
sive borings. 
There is, however, some evidence of the former existence of 
a Miocene lake, which possibly extended during Pliocene times, 
six miles and a half south of Lennox, i.e. in the very bottom 
of the trough, in the eastern bank of Long creek, fine sands and 
gravels were found to form the base of the drift at a level of 
1256 feet above sea. They made the impression upon the writer, 
