486 THE GEOLOGY OP MINNESOTA. 



[Diabase. Porphyryte. 



magnetite grains. The rock is very fresh, even the olivines showing very little 

 alteration. It is illustrated by figure 3, plate II. 



One section examined. 



Age. Cabotian. u. s. G. 



No. 678. DIABASE. 



" This hill extends to Little lake, and along the southeast side, where the superposition of the strata can 

 be made out in descending order, the highest stratum being of the ' red granite.' Thus (witha dip 10 S.W.): 

 " Red granite (not actually seen in place), 



" No. 678, in a regular bed of the thickness of - 15 feet 



" No. 076 [679], porphyry with red feldspar 

 " No. 679A, porphyry with gray feldspar 

 "No. 680, fine-bedded, crumbling, seen 18 " 



" The foregoing does not include all the strata, some being invisible, both at the top and at the bottom." 



The specimens are from the southeast side of Little lake, which is a small lake near the centre of the north 

 line of sec. 8, T. 63-1 W. 



Ref. Annual Report, x, pages 76-78, 96; Bulletin ii, page 114. 



Meg. A dark greenish-gray rock of medium grain, composed of feldspar and a 

 greenish, black substance. 



Mic. The section shows an ordinary diabase. The feldspar is in lath-shaped 

 forms clearly earlier than the augite, which is in irregular grains and in ophitic 

 plates of rather small size for a rock of this grain. The feldspar is near lal>rmlorite. 

 The rock has been considerably altered, the feldspar is in part kaolinized and the 

 augite replaced by a green alteration product (fibrous hornblende and chlorite), 

 although much fresh augite remains. Magnetite is abundant, and apatite and secondary 

 quartz occur in small amount. It is possible that some of the green alteration 

 product represents an original unindividualized base. 



One section examined, which possibly may not have been made from this hand 

 sample. 



Age. Cabotian. u. s. G. 



No. 679. PORPHYRYTE. (Diabase.) 



Same locality as No. 678, which see. 

 Ref. Annual Report, x, page 76. 



Meg. Numerous reddish to brownish porphyritic feldspars occur in a compact, 

 very fine-grained, almost black groundmass. 



Mic. The section consists of numerous crowded /fA/.syw phenocrysts in a rather 

 sparse groundmass. The phenocrysts are much altered and frequently somewhat 

 brown in color; some of them still show polysynthetic twinning and in others the 

 alteration has gone so far that twinning cannot be seen, even if present originally. 

 The groundmass is a confused greenish aggregate largely made up of alteration 

 products. Some small lath-shaped feldspars are present in the groundmass, also 

 much magnetite. Most of the green material is hornblende and chlorite. It is impossible 



