530 THE GEOLOGY OF MINNESOTA. 



[Diabase. Porodyte. 



An analysis of a typical specimen of this porphyryte (No. 797G) is as follows: 



Si0 2 60.32 



A1 2 3 15.80 



Fe 2 3 5.42 



FeO .89 



CaO 4.65 



MgO 5.08 



K 2 1-82 



Na 2 4-09 



P 2 5 .12 



H 2 L67 



Total, 99.86 



In the porphyryte have been seen a few fragments of rock similar to the gray- 

 wacke of the region, and at its contact with the surrounding rock the former is finer 

 grained than is usual, but it does not seem to have altered the rocks with which it 

 comes in contact. It seems very probable that this mass of porphyryte is a part of 

 the same magma which produced the volcanic tuff about Kekequabic lake, but that 

 it solidified before reaching the surface, and at present we have no knowledge in this 

 region of a surface flow of rock similar to the tuff. u. s. G. 



No. 752. DIABASE. 



Same locality as No. 751. 



Ref. Annual Report, x, pages 92, 93; Bulletin ii, page 100. 



Meg. A coarse-grained, dark, greenish-gray diabase, in which much of the augite 

 has altered to hornblende. From a dike forty feet wide, cutting rock No. 751. 



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



" In the thin section the feldspars are mostly cloudy and kaolinized, although 

 the triclinic character of some is observable in polarized light. The augite is bi'n/n/ 

 and largely altered to viridite, which mineral substance replaces part of the feldspar. 

 Some green pseudomorphs, apparently after olivine, were observed, while more or 

 less actinolite, biotite and chlorite were seen associated with the viridite and formed 

 from it. Considerable secondary quartz was noticed associated with the feldspar, 

 while microliths are common both in the quartz and augite. Some ti/>atite crystals 

 were seen, as well as 'leucoxene' and pyrite." 



One section examined. 



Age. Probably Cabotian or Manitou. u. s. G. 



No. 753. POKODYTE. 



S. W. % sec. 14, T. 65 G W. Hill north of the narrows of Ogishke Muncie lako. 

 Ref. Annual Report, x, pages 47, 94, 95; Bulletin ii, page 128, plate XII, figure l.f 



Meg. A very fine-grained, compact, greenish rock containing small hornblende 



crystals. 



*Bullrtin ii, p. 100. 



fin the Truth Ann turf 1!< y*or/,p. 94, and also in Bullet in ii, the locality is by a typographical error given asT. 65-4 W., instead 

 of T. 65-8 W. ; and in Bulletin ii the description is placed under No. 153 instead of No. 753. 



