HISTORY OF LA CROSSE COUNTY.

GEOLOGICAL FORMATION OF THE REGION.

BY DR. J. A. RENGGLY.

The geological formations of La Crosse County are substantially the same as in the adjoining counties of Vernon, Monroe and Trempealeau. Where the bluffs arise over four hundred and five hundred feet above the level of the Mississippi and Black Rivers, the upper layer of rock is limestone, often of a thickness of fifty to sixty feet. Below the limestone and wherever it is absent on account of the incisions in the bluffs, Potsdam sandstone' makes its appearance, more or less impregnated with lime, of greater or less hardness, and consisting of horizontal layers of various thickness and of a volume of from four to five hundred feet, according to the elevation of the bluffs. There is a total absence of other intermixtures to this and the limestone formation, except that the former is often of brownish color from oxide of iron or ochre. On many places, especially where the bluff incisions are low, as for instance, at the mouth of State Road Cooley, the lower base of the Potsdam sandstone is visible, and underneath a layer of caolin' of greater or lesser thickness appears, often as loose sand, and often as an easy crumbling, soft rock, mostly of a glittering white, sometimes of a dirty yellowish-white color. Granite, which, by the geological rules, should underlay this, is, according to our observations, not to be found in the county. It is not unlikely that in some places Niagara shale underlies the Potsdam sandstone, as may be observed in the corresponding bluff parts on the Minnesota side, for instance, at Dresbach extending out to the surface of the Mississippi.

All the above formations appear in horizontal layers and belong to the old Silurian system. In many places of the county, especially in the towns of Hamilton, Barre, Bangor, Burns, etc., petrefactions are imbedded in the Potsdam sandstone, which, with the stone itself, are of the Silurian period. In the lowest strata there are found the little shells " Lingula prima" and "antiqua," as remnants of the most ancient animal creation, whilst in the upper layers of Potsdam sandstone shells of the higher order and tribolites abound. Probably, remains of rhizopods, corals, crinoids, fishes, etc., of the Silurian period are imbedded here, though we have thus far not succeeded in finding any.

It is thus apparent that the geological formation of La Crosse County consists of sediment from the ocean that once covered the North American Continent, or the greater part of the same. Indications of the friction of moving ice-masses upon the rocks, which are so frequent in the State of Wisconsin, are scarcely visible in La Crosse County; but we have found large and small loose stones that bear those marks, of which, however, it is difficult to say whether they belonged to the finding-place at the time of the ice period, or whether they were carried there by earlier or