ARCHEAN OR GRANITIC ROCKS 225 



the river and is seen 10 or more feet above the river. It recurs in out- 

 crops for half a mile down the river from the point, referred to by 

 Wincholl, where the brick were made. It consists of angular grains of 

 quartz and small ferruginous concretions embedded in a kaolin groixnd- 

 niass, which is evidently altered feldspar, etcetera. The rotting of the 

 granite will be discussed in considering the Cretaceoiis. The depth to the 

 unaltered granitic rock has not been determined, but I have mapped the 

 area as that of a granitic rock (see figure 1), where only recent alluvial 

 deposit covers it. 



The area of granitic rock outcrop in the Minnesota valley lies at the 

 margin of the fioodplain of the river, on its left (east) side, opposite 'New 

 Ulm. This is practically unweathered rock and, as well described by C. 

 W. Hall (10, page 25), it is "granitic in texture, of medium coarseness, 

 somewhat porphyritic, and of reddish color, due to the abixndance of red 

 feldspar. It is hornblendic, but contains only a small portion of basic 

 constituents, . . . ." A^eins of white quartz 1 to 8 inches wide 

 occur in it. The freshness of the rock here is due, I think, to the protec- 

 tion of a former covering of quartzite conglomerate which is seen out- 

 cropping still at 150 paces to the east of it. The surface of this exposure 

 reaches to about 800 feet al30ve tide, while that on the Big Cottonwood 

 reaches nearer 850 feet. In the city wells of New Ulni, granite is said 

 to have been struck at the depth of 190 feet — that is, at 650 feet above 

 tide. At Shell's brewery, on the Big Cottonwood, it was not so deep. 

 Evidently the granitic rock has an uneven surface and is yet of wide ex- 

 tent. Presumably it lies also under the Courtland quartzite, but at what 

 probable depth will be considered in discussing that rock. Further, the 

 erosion of the granitic rock and the alteration of its superficial part will 

 be discussed in relation to the Cam1)rian and Cretaceous formations. 

 The age of this rock is here assiimed to be Archean. 



The Courtland Quartzite and the Porphtrt Dike 



The ancient name Bedstone could not 1)e used for the quartzite forma- 

 tion of the Minnesota valley without confusion in terminology and it has 

 not been adopted, although otherwise it would have been a very appro- 

 priate name. The term New Ulm quartzite is found in "North Amer- 

 ican Geologic Formational Names,"^ where its origin is accredited to 

 Winchell (5, volume 2, page xxii). It is, however, not evident that 

 Winchell intended to introduce a new formational name. T^lien he used 

 the words "New Ulm and Pipestone quartzites (Potsdam)" he rather in- 



' F. B. Weeks : Bulletin no. 191, U. S. Geological Survey, 1902, 



