24 Report of the State Geologist. 



to original deposition. It is quite possible that it reaches further south than 

 is indicated on the map. 



To the southward, in Colton, crystalline limestone outcrops in association 

 with the gneisses which here appear. One mile south of the main Potsdam 

 exposure, the brook shows an excellent section of the quartzose gneisses on 

 each bank, along with some curious rocks of very uncertain origin, and in any 

 event profoundly changed from their original condition. The exposure is 

 mentioned here as it is of great interest, yet is an easy one to miss. 



Pierrepont £nd Potsdam. 



The boundary so far as traced in these towns is shown on the map 

 accompanying this report. More detailed areal work would probably 

 necessitate considerable changes as the relationships here are complicated, 

 and the drift very heavy. The rocks of the Grenville series are well shown 

 and possess great interest. 



The Potsdam sandstone in the vicinity of Potsdam shows features of 

 importance, and the accompanying map (Figure 2) has been prepared to show 

 in detail the conditions along the river south of the city. Both east and west 

 of the city the Grenville gneisses reach as far, or nearly as far, north as the 

 city itself. They are even found in the city itself. But the river valley is so 

 heavily encumbered with sand deposits and other drift that the limits reached 

 by these rocks are completely hidden. Along the river, however, outcrops of 

 Potsdam sandstone are found for a distance of six miles south of Potsdam, 

 and probably extend still further in the same direction. The structure 

 is sufficiently well shown to prove that it owes its position and attitude to 

 faulting. 



Northward from Potsdam, the upper beds of the sandstone are shown along 

 the river, with the customary low north-westerly dips, followed by the passage 

 beds, and, just below Norwood, by the Calciferous. These are all with the 

 same dip, and manifestly in a continuous and undisturbed section. At Pots- 

 dam, as shown by N. II. Winchell, gneisses of the Grenville series outcrop in 

 the river and in the city itself.* The heavy drift covering east and west 

 prevents tracing this rock to any connection with Grenville exposures in those 

 directions, but it seems most probable to the writer that it is brought up here 

 by a fault. The absence of all the lower portion of the Potsdam formation in 

 the exposures to the northward, and the high dips and disturbed character of 



*Geol. Surv. Minn. 21st Am. Rep. pp. 103-104. 



These outcrops were not seen by the writer, but a letter just received from Professor Winchell verifies the statements 

 made in his report. The presence of this gneiss is the cause of the rapids in the river at Potsdam. 



