506 GEOLOGY OF CENTRAL WISCONSIN. 



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

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

 whole rook is more or less pervaded by a soft clayey material, and spUts out m large 

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

 arid 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 adjoinmg 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 veiy 

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

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

 over 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 neariy 

 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 obser\-ed 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 Ohiginal Structuke and Amount of Erosion of the Baiuj3Q0 



Ranges. 

 Scale natural, 12,0(X) feet to the inch.' 



The quartzitcs 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 oi 

 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 upperinost layers. 



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



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

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

 scale. 



