318 C. W. HALL — KEWEENAWAN AREA OF EASTERN MINNESOTA 



Cambrian series is practically horizontal, and lies directly on the Ke- 

 weenawan lava flows. The sandstones are quartzose and remarkably 

 pure. They carry but little Keweenawan debris in any section disclosed, 

 save that the lowest layers locally carry pebbles, or, where conglomer- 

 atic, even boulders of diabase, as at Taylors Falls. 



Along the Saint Croix, Kettle, and Snake rivers there are nian}^ ex- 

 posures of these sandstones. While actual contact is not seen, the two 

 formations are in such ^^osition as to leave no doubt whatever of non- 

 conformable super])osition. In sections 16, 20, 21, 29, 30, and 31, town- 

 ship 39, rajige 19 west, and thence interrui)tedly for a distance of 18 to 

 20 miles south, the sandstone lies in horizontal beds along the river bluffs 

 which rise to a height of 200 feet, while beside the water the diabases 

 and conglomerates i)ersist in a series of westward-dipping beds. 



STRA TIQRAPIIIC RELA TIONSIIIPS 



From evidence gathered partly at distant points and })artly from the 

 explorations of the Keweenawan rocks themselves, the continuity of the 

 Keweenawan from Keweenaw point to the Saint Croix River valley is 

 established. The frequent recurrence in the intervening area of the same 

 kinds of rocks with the same structure and stratigraphic relationships 

 seems to be incontrovertible evidence. 



Thk Fault Line 



EXTENT OF THE FA ULT 



Along the western border of the area an entirely unexpected relation 

 of the Keweenawan and Cambrian has been discovered. A fault line 

 has been traced which seems to extend from the north end of the area in 

 a southwesterly and southerly direction as far as the Keweenawan series 

 is known to extend. The junction between the two formations is thus 

 apparently marked by a notable fault (see ])lates 27 and 28), on one 

 side of which the more or less shattered eruptives lie, and on the other 

 a shattered and in places upturned white to pink clean quartzose sand- 

 stone is seen. The quartzose sandstone is younger than the eruptive 

 rocks in northwestern Wisconsin. 



Here the faulting has been observed and described by Professor Grant.* 

 The shattered condition of the diabase along the fault line is especially 

 pointed out. The brecciation which subdivides the rocks into remark- 

 ably fine fragments along the contact zone, but becoming coarser with 



* Preliminary report on the copper-bearing rocks of Douglas county, Wisconsin, p. 17. Pub- 

 lished by the state, 1900. 



