EXPOSURES OP THE KEWEENAWAN . 323 



determined with accuracy ; the estimate, based on as careful measure- 

 ments as were possible, is 2,500 feet for the length of the rapids. 



The lava flows at this point in the Saint Croix river valley show an 

 important feature wanting at Taylors falls. Here, v/ithout an excep- 

 tion, the flow consists of three parts: 1, a finely crystalline, and in 

 places apparently devitrified porphyritic basal layer of a few inches 

 thickness ] 2, a medium grained uniformly crystalline middle portion, 

 and, 3, a distinctly amygdaloidal upper division (see figure 1, page 328). 

 The amygdaloid varies greatly in structure ; usually in the central por- 

 tion of the flow it is compact, the occasional vesicles becoming more and 

 more numerous upward until in many cases, filled with zeolitic minerals, 

 they constitute the major bulk of the rock. 



It is this amygdaloid and the associated conglomerates that are ex- 

 plored so persistently for copper. Small quantities of the metal are 

 found in many places, but those great segregations necessary for the es- 

 tablishment of mining operations are yet undiscovered. 



The especial point of emphasis in the geology of this locality is that 

 the Keweenawan rocks dip sharply and distinctly westward ! Former 

 investigations have placed them in the same series of eastward dipping 

 flows as appear along the Snake and Kettle rivers. Irving,* among 

 others, is led to this generalization from the eastward dip of the rocks 

 on these two streams and the same direction shown on the Saint Croix 

 some 30 miles above the mouth of the Kettle river. It is not maintained 

 here that they are different flows ; on the contrary, they may be the 

 very same, but displacements of a profound character have disturbed 

 the rocks along the west side of the district where the fault line exposes 

 the upthrust edges of thousands of feet of lava flows and associated con- 

 glomerates. These conglomerates, in fact, may be continuous from the 

 east to the west of the district. 



The Cambrian sandstones which overlie the Keweenawan just de- 

 scribed are of a cheerful pink color and usually of a medium texture. 

 About 30 feet above the base there is a layer some 6 feet in thickness, 

 filled with quartz pebbles of 1 to 2 inch in diameter. These })ebbles 

 constitute not more than 3 per cent of the entire rock mass; scattered, 

 as they are, without regularity in their arrangement, they form a very 

 unusual phase of sandstone conglomerate. 



That the horizontal, non-indurated sandstones of Taylors falls, the 

 pink sandstones of the Saint Croix at this locality, the quarry rock along 

 the Kettle river, the Hinckley sandstone, and the 700 feet of light col- 

 ored quartzose sandstones penetrated at Pine City are all of one and the 

 same body of Cambrian sediment, no longer seems to admit of the slightest 



* The copper-bearing rocks of lake Superior, Monograph v, U. S. Geol, Survey, 1SS3, p. 245. 



