360 C. W. HALL — KEEAVATIN OF EASTERN AND CENTRAL MINNESOTA 



contain quartz in large proportion. The grains are largely multigran- 

 ular — that is, worn down from granitic quartz — and partly unigranular. 

 Feldspar in two or three varieties is frequently seen ; so, too, are parti- 

 cles of fine black slate and rounded pebbles of an ancient diabase. That 

 the feldspars result from the degradation of granitic rocks rather than 

 basic eruptives is indicated by their albitic rather than anorthitic habit. 

 Microcline is a less frequent, yet by no means rare constituent, while 

 orthoclase was proved in only two or three instances. 



FiouKE 1. — A Giain of Albite Feldapar. 



The albite feldspar is corroded and changine; into a finely granular uniaxial material. To the 

 right the feldspar is almost wholly altered. From slide 1053, from one of the freshest^pecimens 

 of graywacke taken at Thomson. 



Microscopically quartz appears in even greater proportion than macro - 

 scopically. A considerable proportion of the matrix binding the coarser 

 grains is finely crystalline quartz, bearing every evidence of secondary 

 origin. Again, it is the result of alteration in situ. The rule of distri- 

 bution is, the more altered the rock the more finely crystalline, propor- 

 tionally, is quartz seen to be. The original grains of this mineral can 

 usually be distinguished from the secondary by the frequent occurrence 



