THE ARCHAEAN ROCKS. 



613 



Map of the Lower Narrows op tqb Bababoo. 

 Scale 114 inches to the mile. 



On Sections 23 and 26, T. 12, R. 7 E., Sauk county, the Baraboo river passes the north 

 quartzite range in a gorge known as the Lower Narrows of the Baraboo. Fig 29 



mdioates the topography and points of 

 interest in the -noinity of the Narrows. 

 The passage is nearly half a mile in 

 width, the level bottom extending to the 

 fooc of the chffs on either side. The 

 cliffs rise 400 feet above the river, and 

 show finely the great beds of quartzite, 

 and associated strata. The gorge is 

 much wider than needed by the small 

 stream that now occupies it, and may, 

 as ateady suggested, have been at one 

 time used by the Wisconsin, as the val- 

 ley of Devil's Lake seems to have been. 

 It is unlike the latter valley in having 

 been, in part at least, formed first before 

 the Potsdam period, as indicated by the 

 way in which horizontal sandstone and 

 conglomerate ledges occur around tiie 

 heads of steep ravines that extend down 

 the cHff towards the main gorge. Kg. 

 30 is a section north and south through 

 the west bluff at the Narrows. It is a combination of a paced section made along the 

 west line of sections 23 and 26, and of another, not so carefully measmred, made about 

 40 rods further west. The first follows closely the edge of the cliff, where the quartzite 

 beds are exposed to the southern edge of the ridge, the other runs a little west of north 

 from Mrs. Garrison's house, in the N. E. qr. of Sec. 27, and passes for a long distance over 

 horizontal sandstone and conglomerate layers fiUing an old ravine in the quartzite. The 

 scale of the figured section is a natural one, and the contour mdioated is quite closely 

 that of the range on the westernmost of the two lines. 



Beginnmg with the north end of the section, we find, forming the north face of the 

 range, in bold northward sloping ledges, quartz-porphyry about 600 feet in width. 

 Tliis porphyry (1244, 1252) is for the most part dull-red to pinkish on the weathered 

 surface, which is a good deal altered, often iron-stained, and has generally a wHtish 

 imdercrust. The least altered specimens show a brownish-pink matrix, through which 

 are scattered, very thickly, large facets, up to % mch in diameter, of bright-red cleava- 

 ble felspar, and, more sparsely, minute facets of a white kind. In nearly all speci- 

 mens a few small greenish-black blotches, apparently composed of fine mica scales, 

 occur, as also small iron-stained cavities, which often show linings of minute quartz- 

 crystals. The porphyry is very distinctly bedded, showing an E. W. strike, and a dip 

 of 58° to 60° N. Towards its lowest portions, and higher up on the bluff, it becomes 

 gradually more slaty in character (1245 and 1245a), the felspar facets, though very nu- 

 merous, becoming at tlie same time less well defined, and the surfaces of the lamina 

 becoming covered with a soft greasy mineral. This finally changes to a distinct schist, 1 

 about 80 feet wide, containing a large proportion of the soft mineral, and allied to the 

 greasy quartz-scliists occurring at Devil's Lake, but without transverse cleavage. Con- 

 tinuing the ascent of the bluff southward, quartzite is seen lying immediately under- 

 neath the schist, and forming the body of the ridge to the foot of its southern slope. 

 At first this quartzite is much veined and seamed with reticulating veins of white 

 quartz, in which fine specular iron is occasionally to be seen. , At the summit of the 



' Thi ■ "chist is probahly non-magnesian, like the schists of Devil's Lake, ordiuarily called talcose. 

 Wis Sur. — 33 



