OBSERVED STRIKES OF PRE-CAMBRIAN FOLDING AND FOLIATION 693 



tural trend of the Peiiokee-Gogebic district iii Michigan-Wisconsin is 

 north 30 degrees east; of the Marquette, Michigan, district is east-west, 

 and of the Menominee, Michigan, district is west-northwest. 



Older and younger pre-Cambrian rocks of north-central Wisconsin 

 (Weidman, 1907) show mostly steej) dips with very variable strikes, but 

 a general northeast trend seems to predominate. In south-central Wis- 

 consin (Weidman, 1904) late pre-Cambrian quartzite forms an east-west 

 synclinorium 20 miles long. 



From the evidence presented, it is clear that in the Minnesota-Wis- 

 consin-Miehigan region three districts show strikes in close conformitv 

 with Ruedemann's trend-lines, while in five districts the strikes make 

 high angles with those trend-lines, thus exhibiting sharp differences 

 within this area. 



MISSOURI AXD SOUTH DAKOTA 



In the whole vast area, about one million square miles, between the 

 A]3palachians and the Rockies, scarcely anything is known about the pre- 

 Cambrian rocks structures, so that any attempt to draw pre-Cambrian 

 structure lines through this area must be largely guesswork. 



At Pilot Knob, Missouri (Pumpelly, 1873), a relatively small area of 

 pre-Cambrian rock shows an average dip of only 13 degrees and a strike 

 north 50 degrees west. 



In southeastern South Dakota (Todd, 190.3) later pre-Cambrian 

 quartzites in a number of small areas lie in almost horizontal position 

 (shown on map by a circle), and so they are of practically no significance 

 in our discussion. 



The Black Hills of South Dakota consist of a large core of pre-Cam- 

 brian folded strata and igneous rocks (presumably Algonkian) in which 

 the pre-Cambrian structures are well preserved. The general strike 

 seems to be about north-south (Van Hise, 1890), but there are notable 

 variations, as, for example, in the southern part (Newton, 1880), where 

 there is a northwest strike, and in the northern part (Frazer, 1898), 

 where mica schists strike northeast. 



TEXAS 



Two- areas in Texas furnish reliable pre-Cambrian structural data : 

 One is the Lano-Burnet district (Paige, 1912), in the central part of 

 the State, where pre-Cambrian (Algonkian?) strata are distinctly folded 

 with a northwest strike. The other is the Van Horn district (Richard- 

 son, 1913), in the western part of the State, where pre-Cambrian strata 

 strike northeast. In one of these cases the trend corresponds to Ruede- 

 mann's lines, while in the other it is at right angles to them. 



