206 
Wisconsin Academy of Sciences, Arts and Letters. 
ON A NEW OCCURRENCE OF OLIVINE DIABASE IN 
MINNEHAHA COUNTY, SOUTH DAKOTA. 
By G. E. CULVER and Wm, H. HOBBS* 
Field Notes .— The rock considered in this paper occurs in the south¬ 
eastern part of South Dakota, in Minnehaha county. The country rock 
is the Sioux Quartzite, large outcrops of which occur throughout this 
and adjoining counties. 
The quartzite lies in gentle folds with axes approximately east and 
west. In the vicinity of the diabase the dip is to the south at an angle 
of 8° the outcrop being on the north of the diabase. 
The surface of the latter lies somewhat below the general level of the 
district, in the valley of one of the small tributaries of the Big Sioux. 
The stream has cut a trench twenty-five or thirty feet deep and a mile 
in length directly through the diabase. Whether this represents the 
width or the length of the mass it is impossible to say, there being no 
other exposures. 
No actual contact with the quartzite could be found, but from the fact 
that undisturbed beds of the latter occur very near the diabase, it may 
be inferred that the latter is older than the quartzite, and may have 
been an island in the sea in which the quartzite was deposited. 
On the other hand the diabase gives no indication of having been 
poured out upon the surface. Not only is it completely crystalline but 
the size of the crystals indicates slow cooling. The only way in which it 
seems possible to harmonize these facts is to suppose that the ancient 
rock with which the diabase was once covered had been entirely removed 
before the deposition of the quartzite. Such a supposition must rest on 
very insufficient data at present however. 
No other eruptive rock occurs in either Dakota outside of the Black 
Hills, 300 miles away, but some seventy or eighty miles northeast of this 
locality, in the Minnesota valley, there are many outcrops of eruptives 
in the gneisses and other ancient rocks of that region. In order to com¬ 
pare the South Dakota diabase with these rocks, the best exposures in 
the neighborhood of Granite Palls, Minnesota, were visited and speci¬ 
mens collected representing a dozen or more varieties. Macroscopically, 
none of these bore very close resemblance to the South Dakota diabase. 
* Field notes by Gr. E. Culver and petrograpliical notes by Wm. H. Hobbs. 
