Recent compilation of data by the Kansas Geological Society's Basement Rocks Committee regarding the Precambrian in part of the Mid-Continent provides a framework in which to analyze further this rock complex. In Kansas alone, more than 2,100 wells are known to have penetrated the Precambrian, and approximately 50-60 tests a year are drilled into the basement. Studies are now in progress to attempt to determine detailed spatial relationships of these rocks and their intricate geologic history.

By using only preliminary information, it is possible to differentiate general categories of rock types at the Precambrian surface, which in Kansas is buried beneath Paleozoic rocks at depths from 500 to 9,000 feet. Rocdk types recognized include granite, granodiorite, syenite, diabase, rhyolite, and metasediments; the interrelations of these are exceedingly complex.

Sediments, chiefly alternating silicate-cemented sandstone and indurated shale, have been described from Missouri and may be abundant elsewhere. Outliers of schist capped by resistant quartzite form buried hills in central Kansas.

Diabase and related types of mafic rocks are found; syenite may be in the form of intrusive plugs. Extrusive rocks are represented by the rhyolite and associated suites. Granite of various kinds is by far the most extensively recorded rock type in the Mid-Continent; granodiorite has very limited distribution. In many areas, the deeply weathered and perhaps even reworked pre-Reagan (or Lamotte) basement rock constitutes "granite wash" assumed here to be Precambrian.

Geographic distribution of different rock types is suggestive of pre-Paleozoic structure. A wide band of metasediments through central Missouri, northeastern Kansas, northeastern Nebraska, and southeastern South Dakota forms a large arc convex southwest perhaps outlining the southwestern flank of the old Wisconsin Highlands. Present dip of the metasediments in west-central Missouri is known to be southwestward. Outside the belt of metasediments are igneous rocks and some metasediment outliers.

Potassium-argon ages determined by J. L. Kulp on five samples from Barton, Rush, and Morris Counties, Kansas, yielded dates of 1,165 to 1,460 million years, comparable with ages elsewhere in the central United States.

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