THE ARCH^AN ROCKS. g^^^ 



ing horizontal strata, see Atlaa Plate XIV, and the sections of Plate XXI of this vol- 

 ume. If the view, just mdicated, that there ai-e no folds concealed beneath the ".and- 

 stone in the intervenhig valley, is the correct one, the thickness of the entire series mum 

 be very great, and the amount of erosion that has taken place correspondingly great. 

 Fig. 23 indicates the present structure and relative positions of the ridges, and, by the 

 dotted Knes above, the possible original structure, and the extent of the erosion that has 

 taken place. The figure is drawn to a natural scale, the hue of section bemg the same 

 as that of Pig. II of Plate XX. The heavy black hne represents the overlying Potsdam 

 sandstone. It is not impossible that the valley between the ranges owed its existence, 

 to some extent, m the first place, to soft rocks intercalated between the harder quart- 

 zites. The hypothesis of Pig..23 is not altogether satisfactory. The entire disappear- 

 ance of the other side of the great arch, as well as the peculiar ways in which the 

 ranges come together at their extremities are difiioult to explain by it. It may be said 

 in this connection that the dip observations toward the west are not so satisfactory or 

 numerous as they might be. 



The irregular areas over which the Archaean rocks are at surface, are indicated, as ac- 

 curately as present knowledge will permit, on Atlas Plate XIV. The greatest difiiculty 

 in the tracing of the exact boundaries of the quartzite areas hes m the fact that rem- 

 nants of the horizontal sandstones which flank and cover them may be found at almost 

 any elevation upon the blufi's, so. that no barometrical observations are of avail. The 

 areas, as indicated, are, however, very nearly accurate. There are places withm them 

 where, without doubt, patches of the covormg sandstone occur, but the quartzite is m 

 every such case but a short distance beneath. The pecuhar features of these sandstones 

 and their relations to the other Silurian strata of the region are treated of on a subse- 

 quent page. 



Beginning the detailed descriptions at the best known, and at the same time one of 

 the most remarkable, points about the quartzite ranges, we note first the occurrences in 

 the vicinity of Devil's Lake, in T. 11, ranges 6 and 7 E., Sauk county. Here the 

 southern range is cut entirely tlirough by a deep quartzite-walled valley or gorge, 500 

 feet in depth, and three-fourths of a mile in vridth. In its northern portion this valley 

 trends due north and south for about a mile; turning then abruptly at right angles it 

 extends eastward two miles and a half. In the north and south part hes Devil's Lake, 

 with a length of sometliing more than a mile, and a width one-fourth less than this, its 

 surface being about 100 feet above the vaUey surface at the eastern end of the gorge, 

 more than 100 feet above the Baraboo river at Baraboo, and more than 200 above tha 

 Wisconsin at Merrimack. It is held in this elevated position by two immense morainic 

 heaps of glacial drift lying at either end of the lake and rising more than 100 feet 

 above its level. Tlie lake has a nearly level sandy bottom except near the shores, and is 

 over most of its area some 30 feet in depth. It has no outlet, and but one small stream 

 mnning into it. It is thus probably fed chiefly by springs, and maintains its level by 

 evaporation and by filtering tlirougli the heajis of gravel and sand which hold it in 

 place. Near the northwest corner a small stream running into the Baraboo passes 

 witliin a few rods of the lake, and possibly carries with it some of the lake water. 



As shown on the map of Plate XIX, on its west and south sides the lake washes the 

 bases of the bounding cHffs of the gorge. Both east and west cliffs are highest near 

 their southern ends, towards which they rise gradually from the north, following roughly 

 the dip planes, which also rise southward, and the edges of which can be seen quite well 

 marked on either wall of the gorge. By aneroid measurement the liighest point of the 

 west bluff is 475 feet above the lake level or 860 feet above Lake Michigan, and the 

 southern portion of the east bluff but httle lower. In theu: upper portions the clifl's are 

 vertical, sometimes for as much' as 200 feet or more, but their lower parts ai-e dothod 



