552 THE GEOLOGY OF MINNESOTA. 



[Diabase. Granite. 



the dike and has along it a small amount of the granite. Cleavage faces of feldspar 

 are scattered thickly through the rock and are very evident, owing to the finer 

 grained state of the other minerals, on the contact side of the specimen. 



Mic. The section shows plagioclase, augite, magnetite, hornblende, chlorite 

 and calcite. 



The playiodase occurs in numerous, small, lath-shaped crystals, and also in larger 

 grains of irregular outline. These larger grains are of later date than most of the 

 lath-shaped crystals, but there is not a very sharp distinction to be made between 

 the two forms of feldspar, all gradations from the laths to the irregular grains being 

 seen. Albite twinning, especially in the laths, is common, and pericline twinning 

 is rare. Equal extinction angles on either side of the albite twinning line in sections 

 cut normal to 010 run up to 30 in the lath-shaped crystals, indicating acid lalradorite. 

 The irregularly outlined feldspars are apparently a little more acid than the lath- 

 shaped crystals and are probably amlc^inc. 



The auyitc is later than at least the lath-shaped feldspar and occurs usually 

 in small grains, but sometimes larger areas are found which assume an ophitic 

 relation to the feldspar. The irregular feldspars appear about cotemporaneous with 

 the augite, although this relation is not very marked. The augite is of a light 

 cinnamon color and is slightly pleochroic. It has altered in quite a large measure 

 to green fibrous hornblende and chlorite, with some calcite. 



Magnetite is abundant in small grains and is often enclosed in the augites. 



One section. 



Age. Dike, perhaps of Keweenawan age, in Archean granite. u. s. G. 



No. 803. GRANITE (with hornblende) . 



From an old quarry of Breen and Young, East St. Cloud. 



Ref. Annual Report, x, pages 107, 121; Final Report, vol. i, pages 144, 145, 196-199 (No. 6), plate A, figure .">. 



Meg. A coarse-grained, reddish granite, composed of reddish feldspar in 

 abundance, and quartz and hornblende in smaller amounts. Some of the feldspar 

 is in larger grains than the average mass of the rock, and thus a sub-porphyritic 

 aspect is given to the hand specimen. 



Mic. The main part of the section is composed of large grains of fcl<hj>nr and 

 also of quartz, which, however, do not show crystal outlines. These larger grains 

 interlock with each other and also with smaller grains of feldspar, quartz and horn- 

 blende, which smaller grains form a groundmass, but of small quantity, for the rock. 

 The feldspar is considerably clouded, but appears to consist of orthorliiw, inicroperthite, 

 microcline and oligodase. The hornblende is in small amount, and is greenish to 

 brownish in color. It is strongly absorptive, the ray which vibrates approximately 

 parallel to the vertical axis being almost entirely absorbed. 



