470 THE GEOLOGY OF MINNESOTA. 



[Porphyry te. 



Mic. The section shows a rock of granitoid texture, somewhat altered. The 

 minerals are feldspar, quartz, augite, hornblende, biotite, apatite, magnetite, pyrite and 

 greenish and yellowish alteration products. 



The feldspar is in places considerably altered to a fibrous mass appearing like 

 sericite, and in other places is gray and almost opaque. Albite twinning is rather 

 common and one of the fresher grains gives equal extinctions of 26 on both sides of 

 the twinning line, which would indicate a feldspar at least as basic as acid labradorite 

 with the composition Ab, An, . Another grain cut approximately perpendicular to a 

 gives an extinction angle of 64, and another approximately normal to c gives 4; 

 the first of these angles would indicate a feldspar near labradorite, and the second 

 a more acid one, but neither of these two measurements is entirely satisfactory, as 

 neither section was cut exactly perpendicular to the bisectrix. 



The augite is largely decomposed to a mass consisting of hornblende, biotite, 

 magnetite and confused greenish and yellowish alteration products. The yellowish 

 material is in part of the nature of a stain which has penetrated the cracks in the 

 feldspar. A little unaltered augite is present. 



Quartz is quite common, sometimes appearing in grains which might be original 

 and sometimes among the alteration products of the augite and also intergrown with 

 feldspar (micropegmatyte) and apparently replacing some of the more altered 

 feldspars. 



Magnetite is abundant, and associated with it is a little pyrite. Apatite needles 

 are common. 



Two small sections examined. 



Age. Cabotian. 



Remarks. This rock is now a quartz gabbro and is so designated. But it is not 

 intended by this to affirm that the quartz, or part of it, is original, although such 

 may be the case. On the other hand all of the quartz may be of secondary origin; 

 some of it at least is of that nature. u. s. G. 



No. 649. PORPHYRYTE (with augite'i). 



"Sample of the rock seen in loose pieces along the trail going north between sec. 36, T. 63-1 E. and sec. 31, 

 T. 63-2 E. A high hill runs along the east of the trail, the trail being west of the town line." 

 Ref. Annual Report, x, pages 68, 69; Bulletin ii, pages 78, 79. 



Meg. A brownish-red rock, showing elongated grayish feldspars and irregular 

 elongated dark masses (augite) in a reddish groundmass; but the distinction between 

 the groundmass and the rest of the rock is not very sharply marked. It might be 

 called sub-porphyritic in texture. 



Mic. The section shows many elongated feldspars, which are in general highly 

 altered (but are gray instead of reddened), and augite in a groundmass composed of 



