IOWA ACADEMY OF SCIENCES. 21 



Eruptive Rocks in Iowa. — During the past few years a number of deep wells 

 have been made in different parts of the State. The depths reached are from 

 twelve hundred to two thousand feet. Several of these deep borings are of 

 particular interest as they pass through all the sedimentary rocks into the crystalline 

 basement below, penetrating the latter in some cases to the extent of several hun- 

 dred feet. A typical gray granite has been recognized in some instances; in others 

 different types of eruptive rocks. 



The latest drilling in the northwestern part of the State is the well in Hull, in 

 Sioux county. At a very considerable depth — between seven hundred and fifty and 

 twelve hundred feet — several thick beds of flint-like rock were passed through. 

 The different beds of the rock were separated by gravels or sands several feet in 

 thickness, if the samples and records are to be relied upon. Some of the flint-like 

 fragments were sliced by Mr. S. W. Beyer, of the Agricultural College, and upon 

 petrographical examination proved to be typical quartz-porphyry— a truly igneous 

 roik, very acid in character, essentially identical with granite, but cooling under 

 somewhat different physical conditions. Under the microscope the ground mass 

 appears microgranitic with large clear crystals of quartz and feldspar scattered 

 through it. Both kinds of phajnocrysts have the crystallographic angles rounded 

 through magmatic corrosion. Characteristic embayments are also apparent. Now 

 the interest in these eruptive rocks in situ, true lava flows of perhaps paleozoic 

 date, so far beneath the surface, centers around their nearness to the great mass of 

 metamorphosed sandstone known as the Sioux Quartzite. This quartzite outcrops 

 along the Big Sioux river in the northwestern corner of the State, extending 

 northward from Minnehaha county. South Dakota, to Pipestone and Rock counties, 

 Minnesota. Its geographical extent is much greater than was formerly sup- 

 posed; and it has lately been found well exposed in Iowa twenty miles to the 

 southeastward of the outcrops heretofore noted. There are large exposures in 

 Lyon county in the vicinity of Rock Rapids, where the stone is quarried, and 

 borings indicate a much farther eastern extension. It has been usually believed 

 that no other crystalline rocks appear in the neighborhood of the Sioux Quartzite 

 outcrop. It is of considerable interest then to know that G. E. Culver* has very 

 lately discovered in the midst of the quartzite of southeastern Dakota, in Minne- 

 haha county, within less than half a dozen miles of the Iowa boundary, or more 

 accurately in Sections 15 and 22, R. 49 west, Twp. 101 north, a large mass of 

 diabase. It is exposed for fully a mile along one of the tributaries of the Big 

 Sioux, 



Hobbs.t who has examined the intrusive rock microscopically, finds it to be a 

 well pronounced coarse-grained olivine diabase, with hornblende, biotite, ilmenite, 

 and apatite present, in addition to the plagioclase, augite and olivine. 



The presence of this massive basic rock ot undoubted eruptive origin is very sug- 

 gestive of the agencies that may have been involved, to some extent at least, in 

 metamorphosing the old Sioux sandstone. Further investigations will doubtless 

 disclose other similar types of eruptives in the quartzites of the neighborhood in all 

 three of the states already mentioned. 



The quartzite still has its planes of sedimentation clearly defined. It lies in low 

 folds which are quite noticeable in many places. 



*Culver and Hobbs: On a new occurrence of Olivine Diabase in Minnehaha county 

 South Dakota. (Trans. Wisconsin.Acad. Science, Arts and Letters. Vol. VIII, pp. 208- 

 210. Madison, 1892.) 



tLoc. cit. 



