520 THE GEOLOGY OF MINNESOTA. 



[Conglomerate. 



Mic. M. E. Wadsworth's description of this rock is as follows :* 

 "The section is grayish green and composed of fragments of old basaltic rocks 

 (melaphyr) with some probable andesyte fragments, and a few of other rocks. The 

 basic eruptive material retains its structural characters but has its base and ground- 

 mass altered to a confused fibrous and scaly plexus of green chlorite and colorless 

 micaceous minerals with magnetite dust. The porphyritically inclosed feldspars in 

 the melaphyr and andesyte fragments are largely altered to colorless micaceous 

 scales, but still retain their plagioclastic characters, distinguishable in polarized 

 light. Chlorite, in plates, is also common. Considerable yi/urtz was observed in the 

 rock mass and in larger grains, but it is doubtful if any of the grains are original, 

 but are rather formed by the secretion of silica during the process of the rock 

 alteration. Much pyrite of secondary origin and a small fragment of jasper were 

 seen. There occurs, as au alteration product, in the section, some microliths and 

 radiating small bluish or brownish crystals. They are generally associated with 

 quartz or pyrite as the nucleus for radiation, and are uniaxial, negative, varying in 

 dichroism from a greenish blue to a brownish yellow. They are here referred to 

 totintHiline. This mineral is not given in Prof. Winchell's list of minerals found in 

 Minnesota, published in the annual report for 18S2, and no mention is made of it in the 

 index to Irving's Copper-Bearing Hocks." Two sections examined. 



4 



Age. Keewatin; quite probably Upper Keewatin. u. s. G. 



Remark. It is, of course, almost impossible to decide from the hand sample and 

 the sections whether the matrix of this rock is a volcanic tuff or whether it is 

 fragmental material derived by erosion and decay from the terrane of " greenstone " 

 immediately adjoining (see remarks under No. 731). Either origin is possible, but 

 from the fact that only a short distance to the northeast (West Sea Gull lake) and 

 also to the southwest (Ogishke Muncie lake) a true conglomerate is found, which 

 is clearly younger than this " greenstone " terrane, this rock is here called a conglom- 

 erate. We see no reason why some of the quartz grains in the rock may not be 

 fragmental grains, dating from the deposition of the rock. 



Tourmaline has since been found at other localities in the state, as in the granite 

 at Koochiching falls on the northern border of Itasca county (No. 1030G);f also in 

 Nos. 773, 1525A, 2162. u. s. G. 



No. 738. CONGLOMERATE. 



Near the northeast end of Ogishke Muncie lake; probably in N. W. '4 S. E. % see. 13, T. (io-G W. 

 Ref. Annual Report, x, pages 90, 91, 95; Bulletin ii, pages 122, 123. 



Meg. A light-gray rock, containing much quartz and feldspar and also small 

 fragments of different rocks. 



Bulletin U, pp. 121, 122. 



tA. N. WINCHELL: American Geologist, vol. xx, p. 296. 



