SCENIC FEATURES OF THE MAGNESIAN TERRANE. 343 



Localities of (hi 1 Magnesian Series. — In the Minnesota river valley the 

 rocks of this series extend continuously from Judson to Shakopee. 

 Some exposures are at a distance from the stream and others lie in the 

 banks of its tributaries, as along the Blue Earth; on the Saint Croix 

 from the neighborhood of Marine to Point Douglas are many conspicu- 

 ous exposures; on the Mississippi the most northerly masses arc above 

 Nininger and Langdon, whence they are continuous in a succession of 

 rugged and castellated bluffs, usually capping the Potsdam, to the Iowa 

 line: the tributaries of the Mississippi, the Vermilion, the Cannon, the 

 Zumbro, the Whitewater, the Kollingstone and the Root present many 

 faces of these rocks; and the sections of many artesian and deep wells 

 throughout southern Minnesota. 



Structural Characters. — Structurally, this series varies more than any 

 other within the Paleozoic of the state. This arise- from the varied 

 character of the rocks. The dolomitic portions are massive, and form 

 those striking scenic effects seen along the streams whose gorges are cut 

 into or through them. Its thickness is considerable; its walls, through 

 weathering and corrasion, have been gnawed away until they stand far 

 apart and face each other with rugged, hoary and castellated fronts. 

 Trickling waters have produced their effect in moulding the faces of 

 these walls, or, as in the driftless area* they have removed large masses 

 of the rock, thus, producing chasms, into which has fallen some debris. 

 In this manner the many sink-holes have been formed which are to be 

 seen on the otherwise smooth prairies of this area. 



In many localities a brecciated condition is present in the dolomites — 

 ;i condition not infrequent in Wisconsin, according to Chamberlin.f 

 Ordinarily the chips composing this breccia are not large. In Winona 

 county a brecciated structure characterizes much of the Saint Lawrence. J 

 This even appears in some of the silicified material in the upper Shako- 

 pee. Another feature almost everywhere found in the central lied Cthe 

 lower Shakopee) is a geodie and concretion;) ry tendency. Silica is thus 

 collected into segregations of great purity. Redwing and vicinity may 

 be taken as ;i typical locality. In the dolomitic mass forming Lagrange 

 mountain, now for some reason more popularly called Barn bluff, there 

 are numerous segregations of a. light gray microcrystaUine silica, together 

 with partial fillings which show cavities not infrequently of Large size, 

 witb walls covered by sparkling facets of quartz crystals. In places these 

 crystals arc amethystine and of considerable size. An oolitic phase i- 



*The Driftless irea of the Upper Mississippi, Chamberlin ind Salisbury, 6th Ann. Rep. U. S. 

 I. Survey, 1885, p. 205. 



t Geology of Wisconsin, vol. i, 1883, p. 140; vol. ii, 1877, p. 278. 



fWinchell, N. EJ. : Geology of Minnesota, Final Report, vol. i, 1884, p. 264; ef. G ology^ol [owa, 

 pt. i, L858, p. 333. 



