506 GEOLOGY OF CENTRAL WISCONSIN. 



and at the Upper Narrows of the same stream. These are white to straw-colored, dis- 

 tinctly granular in texture, the quartz grains being of translucent glassy quartz. The 

 whole rock is more or less pervaded by a soft clayey material, and splits out in large 

 thin sheets. On the northernmost portions of the north range, at the Lower Narrows, 

 and also for a short distance to the westward, a great thickness of quartz-porphyry is 

 to be observed. This porphyry resembles that of the several small porphyry areas of 

 the adjoining portions of Columbia, Marquette and Green Lake counties and proves at 

 once that we must regard these areas as part of the same formation that appears in the 

 Baraboo ranges. 



In the quartzite, milk-white veins and nests are frequently to be seen. In some 

 places, as at the Upper Narrows, the white quartz veins show frequently geodic cavities, 

 lined with quartz-crystals of great clearness and beauty, and not unfrequently of very 

 large size, though usually small. In the veins at the Upper Narrows, such crystal - 

 lined cavities are exceedingly numerous. Along with the crystals, sometimes compacted 

 oyer them, sometimes loose in the cavities, and again in thin seams by itself, is to be 

 seen a soft, white mineral. This is often pulverulent, at times gritty, at others a nearly 

 impalpable powder, and is shown by analysis to be essentially a silicate of alumina. 

 With the white quartz, in nests of some size, is often to be observed brilliant specular 

 iron in large crystalline surfaces. It occurs also in some of the layers of quartzite, in 

 fine scales. Titanic iron is also reported. These, with the peculiar aluminous silicate 

 alluded to in connection with the quartz-schists, are the only minerals known to occu' 

 in the Baraboo rocks. 



FIG. 23. 



IDEAL SKETCH, SHOWING ORIGINAL STRUCTURE AND AMOUNT OF EROSION OP THE BARABOO 



RANGES. 

 Scale natural, 12,003 feet to the inch. 1 



The quartzites and associated rocks are quite distinctly bedded, though the bedding 

 is not unfrequently obscured by cross-jointing, which is often to be observed on a grand 

 scale. The dip, wherever observed, is towards the north, through the whole extent of 

 both ranges, but varies much in amount. In the southern range it is usually quite 

 low, as low sometimes as 15 in the middle and broadest portions. In the northern 

 range the dips are always much higher, running from 55 to 90. The rocks of the 

 two ranges appear, however, to be parts of a continuous series, the quartz-porphyry 

 beds of the northern range constituting the uppermost layers. 



For the relative positions of the different ranges and their relations to the surround- 



1 A sketch, similar to this, accompanies a paper by Prof. Ohamberlin, " On the Method of Up- 

 heaval of the Baraboo Ranges," Trans. Wis. Acad. Sci., Vol. II, but it is not drawn, on a natural 

 scale. 



