Stoux City Academy of Science and Letters. 153 
GEOLOGIC HISTORY. 
The rocks exposed in Dakota County furnish few and 
disconnected chapters of the geologic record and it is 
necessary to trace the missing stages of sedimentation in 
the rocks of surrounding localities. Less than 100 miles 
to the north, in the vicinity of Sioux Falls, South Dakota, 
are numerous exposures of a hard, siliceous rock pink in 
color, known as the Sioux quartzite. Deep borings radi- 
ating from Sioux Falls as a center have shown that the 
surface of the quartzite is exceedingly irregular, but 
that a broad underground ridge of the rock extends 
northeast-southwest through Pipestone, Minnesota, and 
Sioux Falls, South Dakota. This quartzite is in all 
probability of pre-Paleozoic age and it has been referred 
to the Algonkian. Its derivation has been traced to a 
probable Archaean land surface composed of granite 
and schist in central Minnesota which possibly extended 
westward to South Dakota. From this land surface 
material was derived by stream action and wave erosion 
and laid down as shore deposits of sand and thin clay 
beds. Slight volcanic disturbances seem to have 
occurred with igneous outflows after the deposition of 
the Algonkian sandstone, as the presence of a dike or 
quartz-porphyry has been noted at Hull, lowa, a dike of 
olivine-diabase at Corson, South Dakota, and the pres- 
ence of what was thought to be similar rock in borings 
at Yankton, and Alexandria, South Dakota. 
Most recently there has been discovered at Sioux 
Falls, South Dakota, an exposure of gabbro or diabase 
containing much magnetite. Its relation to the quartzite 
has not yet been ascertained but it is thought to be intru- 
sive. 
Subsequently the sandstone underwent metamor- 
phism by silicification to an intensely hard and vitreous 
quartzite, and the clay beds were changed into pipestone 
and siliceous red slate, both of which are found in south- 
west Minnesota. Microscopic examination of the quartz- 
ite shows that silicification was effected by crystallization 
of quartz around the separate grains, filling all inter- 
vening spaces and producing so compactly cemented a 
material that in breaking the rock the fracture divides 
aTodd, J. E., The newly discovered rock at Sioux Falls, S. Dak: 
Am. Geol. Vol. 33, No. 1, 1904. 
