Annual Report - Geological and Natural History Survey of Minnesota Volume 19 (Paperback)

This historic book may have numerous typos and missing text. Purchasers can usually download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1892 edition. Excerpt: ...large erratic blocks cover all of the older sedimentary rocks. Suddenly the broad deeply eroded valley stretches out before the observer and a surprising view is given. With the greatest regularity several terraces rise one above the other and on the Wisconsin side at the same altitude can be readily followed as a step-like descent to the bottom of the valley. The upper terraces are washed out of the diluvial formation; the lower ones lying partly in the Lower Silurian dolomyte, partly in the Potsdam or St. Croix sandstone which was formerly distinguished by American geologists as an older member of the Lower Silurian. Along the upper St. Croix are added the great eruptions of melaphyr and melaphyr-porphyry which form the base of the Silurian system in the vicintiy of lake Superior and which I shall have opportunity during this work to revert to more in detail. On a geological map of Wiscon sin published in the year 1869 by I. A. Lapham of Milwaukee, in place of these four formations referring to Owen's map of 1851 there is but a single formation "trap," which branches from the southwestern point of the granitic and metamorphic rocks of Wisconsin. This comprehension is the proper one, as we shall see shortly that the crystalline rocks of the St. Croix are older than the Silurian sandstone. Similar formations of that which American geologists call "traprock" are found plentifully embedded in and on the edge of the Archasan formation in Wisconsin, particularly in conjunction with quartzytes and conglomerates of Huronian age. Owen regarded these rocks of the St. Croix as 'porphyritic trap" and compared them with the Norwegian porphyry as found on the western side of Christiana-fjord at Bogstadt. I also regarded it...