PETROGRAPHY OF TEE KEWEENAW AN 649 



A large part of the ground-mass is now orthoclase in one or two sec- 

 tions examined, but many other minerals are associated: quartz, 

 calcite, chlorite, etc. 



Porphyritic variations. — In Minnesota, phenocrysts from one-half to 

 two inches long are developed in the conchoidal and hackly fracturing 

 types at several localities. At Taylors Falls they are red and else- 

 where usually gray. The fresh samples are labradorite, but altera- 

 tion had the same effects on these large crystals that were revealed 

 in the smaller ones. As the grain becomes dusty and granular, 

 extinction becomes wavy, twinning bands disappear, and if there is 

 an aggregate extinction the angle is low. A hard rock of this type 

 can be followed along the strike for some miles on Kettle River. Two 

 samples from widely separated points differ from each other so little 

 that only the average is given in Table IV. The extreme was a 

 variation from 6.05 to 6.89 per cent of lime. The phenocrysts have 

 not affected the composition to make it differ much from non-por- 

 phyritic types. 



Glasses. — ^A few inches at the base of each flow is commonly of 

 the appearance of devitrified glass, and the immediate region of the 

 amygdules at the tops of flows is similar. The alteration is advanced, 

 though the crystallization is never very coarse. The conchoidal 

 fracture remains, and in plain light the sections reveal microlites in 

 a translucent field, and a few show flow structures. 



Amygdaloidal textures. — ^Most of the flows have an upper zone, 

 not sharply distinct from the main body, but characterized by 

 increasing numbers of cavities, around which the rock is more glassy 

 than elsewhere. Such porous glass, chemically unstable and physi- 

 cally weak, and affording good channels for water-circulation, now 

 contains such a variety of secondary minerals that it seems hard at 

 first sight to assign them all to the same rock for origin. Some 

 cavities are now open — ^probably from recent leaching; some are 

 filled with a single mineral; some contain three or four minerals in 

 complex relation to each other. The series of minerals in order of 

 formation has been generalized in the Michigan reports, and is 

 essentially correct in this area. Table IV includes two analyses of 

 Minnesota amygdaloids, but they are not in this particular 

 Keweenawan area. 



