Ti GEOLOGICAL SURVEY OF WISCONSIN. 



can be drawn from analogy. It is sufficient to remark here provision- 

 ally, that the apparent common lithological character and the same 

 general trend of the trap ridges crossing the St. Croix river would 

 point to the same geological era. 



A somewhat careful examination at St. Croix Falls enabled the ex- 

 ploring party to determine that the Potsdam sandstone was deposited 

 in the ancient Silurian sea at a period subsequent to the formation of 

 the trap, whatever may have been its origin. The beds of the Pots- 

 dam sandstone are horizontal over the uneven and tilted surface of 

 the underlying igneous or crystalline rocks. 



Almost in contact with the trap, the sandstone contains numerous 

 well preserved organic remains. Three miles north of Osceola Mills, 

 a ledge of sandstone was found lying horizontal, unconformably on 

 the more ancient formation. These facts incontestably prove, that 

 this particular trap dyke was not erupted or upheaved through n, 

 superincumbent layer of sandstone. 



At Kettle river rapids was first found, in ascending the river, a red 

 sandstone, having all the lithological characteristics of the Lake Su- 

 perior red sandstone, in talus along the shore. The water in the river 

 v/as extremely low, so that unusual opportunity was afforded for ob- 

 servation. The slabs and fragments of the red sandstone were sharply 

 angular, showing that they were in situ or not far away. In the 

 bank, forty or fifty feet higher, was a fine exposure of Potsdam sand- 

 stone in a massive ledge. 



The party had neither the time nor the means at its disposal to 

 make an excavation to ascertain by definite observation whether the 

 Superior red sandstone existed in true formation beneath the Potsdam. 

 Such a fact, definitely settled by a competent observer, would be 

 strong evidence that the former is, at least in part, older than the 

 latter. 



At " Pine Island," in the same rapids, the Superior red sandstone 

 was found in ledge, bearing abundant angular fragments of the adja- 

 cent trap, forming a breeciated conglomerate that is evidently kindred 

 to the conglomerate that extends from Keweenaw Point, in Michi- 

 gan, along the northern base of the Porcupine and Penokee mountains, 

 west south westward, till it is lost beneath the heavy drift of north- 

 western Wisconsin. Everywhere this conglomerate is formed by 

 fragments of the more elevated liuronian or trap ridges, carried down 

 by the action of the elements and imbedded in the Superior red sand- 

 stone. The ledge of conglomerate under consideration was found to 

 be lower than the horizon of the neighboring Potsdam. Placino- tl^e 

 conglomerate and the Superior red sandstone in the same geological 



