THE ARCH^AN ROCKS. 



515 



FIG. 31. 



shown in the Narrows section is indicated. On the south side of the S. E. qr. of Sec. 16, 

 the porphyry reaches its northernmost point, showing in a bold rocky projection. The 

 rock here (1,242) presents a dull-brownish appearance on a weathered surface, and is 

 much fissured by weathering, the surfaces of the fissures showing generally a brownish 

 iron stain. A schistose structure is apparent in places, and the bedding is plain, with an 

 E. W. strike, and dip of 55 N. A fresh fracture shows a compact, flaky matrix, of 

 dark- brown to nearly black, sometimes grayish color, the last being characteristic of al- 

 tered portions. The color is not quite uniform, but is mottled with fine strings and 

 specks of whitish or pinkish color, and of indefinite outline. This matrix fuses easily to 

 a black glass. In altered specimens it is easily scratched by the knife, in unaltered ones 

 the knife makes almost no impression. Sparsely scattered through it are minute white 

 and pink felspar facets, and still fewer large brick-red ones. In this regard the rock is 

 quite different from that of the Narrows section. 



About one-eighth to one-fourth of a mile eastward from this rocky point, in the N. E. 

 qr. of the N. E. qr. of Sec. 21, the porphyry is seen again in a large exposure, showing 

 the same weathered appearance, and bedding structure. Specimens from this place 

 (1,248) resemble the rock last described, having somewhat more numerous felspar facets, 

 and containing: silica, 71.24; alumina, 12.20; iron peroxide, 1.71; iron protoxide, 5.44; 

 lime, 0.98; magnesia,, 0.13; magnanese oxide, 0.97; potash, 1.86; soda, 4.29; water, 0.81 

 =99.63. The large content of soda, as compared with potash, is noteworthy. 

 The quartz-porphyries have thus been traced along the north flank of the range from 

 the Baraboo Narrows, in Sec. 23, to the south side of Sec. 16. 

 Judging from the bedding directions, their whole width can- 

 not be less than three-fourths of a mile, nor their actual thick- 

 ness short of 3,200 feet. They are found nowhere else in 

 the Baraboo region. From the descriptions and analysis given 

 it will be seen that these rocks have a matrix too sih'cious to 

 be purely felspathic, through which are scattered crystals of 

 orthoclase, possibly also of a soda felspar, the porphyritic 

 quartz crystals generally characteristic of quartz-porphyry be- 

 ing absent. They are evidently nearer to true quartz-porphy- 

 ry, however, than to the non-sihcious porphyrites. 1 



Further west again, and until we reach the Upper Nar- 

 rows of the Baraboo, in the town of Excelsior, the quartzite 

 exposures along the north range are only occasional, as on the 

 low ridge north of Baraboo, and on the N. E. qr. of Sec. 33, 

 T. 12, R, 6 E. 



On Sees. 28 and 29, T. 12, R. 5 E., in the town of Excel- 

 sior, the Baraboo river breaks southward through the north 

 quartzite range in a narrow gorge, 200 feet in depth, and 

 something more than half a mile in length, known as the 

 Upper Narrows of the Baraboo. For most of its length 

 e the ravine is just wide enough to admit of the passage of the 

 river, railroad and a wagon road. Here the quartzite core of 

 the north range is finely shown, flanked on either side, and even overtopped, by the 

 horizontal sandstone and conglomerate. Fig. 31 is a map 5 showing the shape and 

 character of the gorge. Fig. 32 is a section on the line A B C of Fig. 31, drawn to a 

 horizontal scale of 750, and a vertical of 300 feet to the inch. 3 



i Von Cottas I.ithclogy, pp. 168 and 214. 

 4 Copied from M;-. W. H. Oanfleld's map of Excelsior. 



' In Fig. 32, the inclined lines in the " veined quartzite " are meant for shading only, and do not 

 indicate any structure to the rock, which, as described, is without distinct bedding, 



UPPER NARROWS OF THE 

 BARABOO. 



Scale, iy 2 inches 

 mile. 



