Now it is quite obvious that a stream of water moving at a rate of three or four feet per day, or even fifty or sixty feet per day, would not develop perceptible upward currents, and certainly would not lift the lightest silt from its bottom. I do not think there are any theoretical grounds for believing that internal glacial currents are developed, which flow from base to surface, carrying bottom debris to the top. One of the most remarkable expressions of the drift phenomena of the Upper Mississippi region consists of belts of boulders stretching for great distances over the face of the country, and disposing themselves in great loops after the fashion of the terminal moraines of the region with which they are intimately connected. Besides this, there are numerous patches of boulders of more or less irregular form and uncertain relations. The whole of these have not been studied in detail, but a sufficient portion of them have received careful examination to justify the drawing of certain conclusions from them. Those which have been most studied lie in Ohio, Indiana, Illinois, Michigan, Wisconsin, Iowa and Dakota. Those of the first three States have been most carefully traced and their constitution is such as to give them the greatest discriminative value. To these our discussion will be limited chiefly.[11] [11] Parts of these tracts were long since described by Bradley of the Illinois Survey. (Geol. Surv. Ill., Vol. IV. p. 227). Collet of the Indiana Survey (An. Rep. 1875, p. 404) and Orton and Hussey of the Ohio Survey (Geol. Surv. Ohio, Vol. III., pp. 412, 414 and 475). The relationship of these tracts to morainic lines and to each other I worked out some years since (Third An. Rep. U. S. G. S. pp. 331, 332, 334) but I owe many details and some important additions to my associate, Mr. Leverett.