154. Sioux City Academy of Science and Letters. 
a grain of sand as easily as it separates the grains fron: 
their matrix. 
Elevation of this quartzite above the sea must have 
followed, and while a land area it suffered great erosion 
as is evidenced by many buried valleys and the generally 
uneven surface revealed by drilling. While a land area 
this quartzite formed a high peninsula or island, which 
in addition to being degraded by erosion slowly sank into 
the sea so that of the sedimentary deposits surrounding 
it, the younger are nearest the center. The depth of this 
pre-Paleozoie quartzite in Dakota County is approximate- 
ly the depth at which it was encountered in the deep well 
at Sioux City, where a thickness of 15 feet was found 
lying upon a micaceous schist at a depth of 1,525 feet. 
Paleozoic history in this immediate vicinity is still 
shrouded in uncertainty. Toward the east the limestones 
and associated beds of the Carboniferous and earlier | 
* Paleozoic appear. Twenty-five miles to the south of the 
county rocks of the upper Coal Measures pass beneath 
the Cretaceous. That there must have been at some time 
during Carboniferous times a shore line bordering the 
Sioux peninsula is probable, and a series of limestones, 
shales, and sandstones between the lower limit of the 
Cretaceous and the upper limit of the quartzite has been 
found in borings at Ponca, Nebraska, and Sioux City, 
Iowa. Whether these beds represent a portion of the 
Carboniferous system or still earlier beds of the Paleo- 
zoic can not definitely be stated. The opinion, however, 
is growing that they represent the upper Coal Measures 
or Des Moines beds of the Iowa geologists. No traces of 
Paleozoic rocks other than these have been noted as un- 
derlying northeast Nebraska, and no Triassic or Jurassic 
formations, so it is assumed that between late Paleozoic 
and middle Mesozoic time the region was a land area 
which suffered heavy erosion. By this erosion the great- 
er part of the Carboniferous beds, if they were once pres- 
ent, would have been removed. 
In the Black Hills more than 300 miles northwest of 
this area several hundred feet of strata of marine origin 
representing nearly all of Paleozoic time are present, 
which indicates that a shore line of the Paleozoic sea 
must have extended north and south somewhere east of 
the Hills. 
