26o EDWARD STEIDTMAN 



The' Baraboo quartzite district consists of an asymmetrical syn- 

 clinical fold of pre-Cambrian formations, striking a little north of 

 east and west, with a steep north limb and a gently dipping south 

 limb. (See Fig. 2 of north-and-south section.) The succession 

 from the base up is igneous rock, unconformity, quartzite, slate, 

 iron formation, and dolomite. The only exposures are those of 

 igneous rocks and quartzite, the latter making a prominent ridge 

 encircling the district. The quartzite ridge of the north limb is 

 known as the North Range, while that of the south limb is called the 

 South Range. 



Several months were spent in the field during 1905-6 in making 

 several thousand observations on dips and strikes of beds and sec- 

 ondary structures, in an area comprising 5 square miles in the vicinity 

 of Devil's Lake and the Lower Baraboo Narrows. Since then the 

 problem has been reviewed annually by the writer, both in the 

 field and in the office. Both data and results have received the 

 criticism of C. K. Leith, to whom the writer is indebted. 



SECONDARY STRUCTURES RELATED TO THE FOLDING OF THE RANGE 



The secondary structures in the pre-Cambrian series of the Baraboo 

 quartzite range consist of (i) closed, interrupted joints parallel to 

 the bedding; (2) strike joints which intersect one or more beds and 

 offset between the beds, and constitute a series of overthrust faults 

 of minute throw; (3) strike cleavage both parallel and diagonal to 

 the bedding. 



The abundance of the joints related to the folding is dependent 

 upon the inclination of the beds, the distance from the contact of 

 formations, and the strength of the beds. They are more abundant ' 

 where the strata are highly tilted, than where their dip is nearly 

 horizontal. Thus in the quartzite of the South Range, having a 

 dip of from 5-20 degrees north and striking N. 65-75 degrees east, 

 only about 20 per cent, of the total number of joints are related 

 to the folding; while on the North Range, where the quartzite strikes 

 E.-W. and is vertical or nearly vertical in dip, 40 per cent, or more 

 of the joints are connected with the folding, and of these, the joints 



I Samuel Weidman, "Baraboo Iron Bearing District," Wisconsin Geological and 

 Natural History Survey, Bulletin No. IJ, 1904. 



