1893.] The Titanotherium Beds. 213 
“ This Miocene deposit is situated about half a mile south- 
easterly from Lead City, South Dakota, and nearly five miles 
south of the city of Deadwood, at an elevation of about 5,200 
feet above the ocean. . . . . Of the extent of this tertiary 
deposit very little is known. It does not outcrop at the surface 
and appears to occupy a small, deep channel or basin eroded 
in the Archean slates which outcrop on all sides within a 
radius of 1000 to 3000 feet. . . . . This occurrence is 
peculiar not only in forming an outlier to the great Miocene 
deposits and in occupying a basin eroded in a range of steep 
Archean hills where lakes are now unknown, but also in the 
nature of the deposit itself, which so closely resembles, in 
appearance at least, the deposits of the same age on the plains. 
The clays of the latter were probably derived from Cretaceous 
shales, but the sediments filling this lake would naturally be 
expected to be very different, the rocks in the vicinity being 
Archean slates capped by Potsdam sandstone and shales, and 
locally overlaid by masses of eruptive post-Cretaceous por- 
phyry 
That this ennai lake was, during Miocene times, entirely iso- 
lated from the great body of water to the eastward can hardly 
be questioned. The similarity just referred to by Professor 
Jenney as existing between the deposits of the small lake and 
those of the larger one is probably due to the materials com- 
posing both having been derived largely from a common 
source. The prevalence everywhere throughout the Miocene 
of large quantities of sands and conglomerates, composed of 
coarse grains or pebbles of quartz, feldspars, and mica, is suffi- 
cient evidence that the materials composing the beds of the 
Miocene were derived largely from sources other than the 
Cretaceous shales, which contain little sand and are always 
very fine grained. The coarser sands and conglomerates were 
doubtless derived from Jurassic, Paleozoic, and Archean rocks, 
and a considerable portion of the Miocene clays probably owe 
their origin to the decomposition of the slates of these older 
_ formations and the kaolinization of granitic feldspars. 
