52 THE JOURNAL OF GEOLOGY. 



Theoretically, as I understand, a stream moving in a straight 

 course on a perfectly smooth bottom would not develop an 

 upward cross current. Each lower layer would move slower 

 than that above it by reason of basal friction, but they would 

 move on in parallel lines. But if irregularity of bottom be 

 introduced the parallelism is obviously destroyed, and if the 

 velocity be high so that the momentum of the particles becomes 

 great relative to their cohesion, irregular internal movements 

 will result, and these will often be of a rotary nature in vertical 

 planes bringing the basal parts of the fluid to the surface or the 

 reverse. For this reason rapid streams abound in rotary currents, 

 while slow streams do not. 



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 examina- 

 tion 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 con- 

 stitution is such as to give them the greatest discriminative 

 value. To these our discussion will be limited chiefly.' 



I Parts of these tracts were long since described by Bradley of the Illinois Survey. 

 (Geol. Surv. 111., Vol. IV. p. 227). Collet of the Indiana Survey (An. Rep. 1875, 



