Process of Formation of Certain Quartzites

In the extreme north western corner of Iowa there is a small area of very hard, thoroughly vitreous rock, which has been known for more than a quarter of a century as the Sioux quartzite. The mass is also well exposed in the adjoining portions of Minnesota and South Dakota. The Sioux granite, as it is now locally called among quarrymen, is of considerable interest for the reason that it has long been the only altered formation known within the limits of Iowa. The apparent metamorphic characters of the quartzite beds is all the more remarkable from the fact that the rocks of the State are so horizontal in their position, so undisturbed by mountain making forces, and so unchanged in lithological characters that it is commonly supposed that all the strata of the State are essentially the same as when deposited in the quiet waters of the great interior sea which once occupied the heart of the American continent