AKCltEAisr Oit GRANITIC ROCKS 223 



The geologic field about the Bedstone and the neighboring parts of the 

 Minnesota and Big Cottonwood valleys has proved to be an inviting 

 one. Each of the geologic formations demanded more complete investi- 

 gation than had been accorded them before, partly by reason of advance 

 in knowledge of correlative rock formations in neighboring regions which 

 have been studied by geologists since the above-mentioned works were 

 published and partly by reason of discoverable new evidence and needed 

 correction of previoiis observations. Advance, which has been making in 

 the knowledge of the "Iron ranges" and of the Baraboo, Wisconsin, re- 

 gion in particular; the changed interpretation of the Dakota sandstone 

 formation, and the question of distinct glacial periods demanded review 

 in relation to this region. Some rock exposures which had been called 

 Cretaceous are found to be altered Archean granitic rock. A volcanic dike 

 has been newly discovered in the Courtland quartzite at Bedstone, and 

 this affords good evidence on the question of structure and induration of 

 that rock. Quartzite conglomerate is found to be Cambrian and not con- 

 temporaneous with the Courtland quartzite. The Cretaceous strata of 

 wide extent are found to be fiuviatile and lacustrine in origin rather than 

 the product of the "Cretaceous ocean." The glacial deposits show two 

 distinguishable till sheets with evidence of pre-glacial and inter-glacial 

 valleys corresponding to the present Minnesota. As already indicated, 

 the Bedstone quartzite is of paramount interest in itself, besides having 

 an important relation to the other formations. The accompanying 

 geologic map (figure 1) shows the extent of the geologic formations. 



The Archeak or granitic Bocks 



Two areas of granitic or gneissic rock appear in the region about Bed- 

 stone. One of these is on the Big Cottonwood river and the other on the 

 Minnesota. The former is the larger, but has not been before described 

 excepting by Upham and AVinchell. Upham refers (6,^ volume 1, page 

 574) to it as a Cretaceous clay forming the base of a "section on the Cot- 

 tonwood river south of New TJlm," which he cites from Winchell. It is 

 described as follows : 



"4. Fine somewhat gritty clay, largely aluminous. This is white and, when 

 long submerged, soft and fluid-lilie, but when dry has to be quarried by blast- 

 ing. This, mixed at the rate of two-thirds with one-third of No. 3, makes a 

 fine, white firebrick — seen 12 feet." 



This rock is in truth an altered or rotted granite in situ, rather than a 

 Cretaceous clay. It has been bored into about 15 feet below the level of 



' Figures in italic refer to citations at the end of this paper. 



