334 C. W. HALL KEWEENAW AN AREA OF EASTERN MINNESOTA 



the breccia of Ta3^1ors Falls is its completely altered condition. It re- 

 quired the close discrimination of microscopic investigation to identify 

 it. It had never been detected prior to Doctor Berkey's investigations 

 b)'' any of the geologists wlio explored the district. Breccias and tuffs 

 are closely associated. They form a bed about 20 feet in thickness at 

 its maximum. Their compacted character makes it difficult without 

 close inspection to distinguish them from ordinary diabase. Volcanic 

 breccias and tuffs will doubtless prove rather frequent rocks in the Lake 

 Superior region. Clements describes them in the Cr3^stal Falls district 

 as attaining a thickness of 500 feet, and still no means was afforded of 

 determining their total thickness.* 



The eruptives display considerable variety of rock characters, due to 

 their genetic habit — that of a series of lava flows. The compact and 

 amygdaloidal phases with their several structural types have already 

 been mentioned. The former is of a granitic to porphyritic, granular or 

 ophitic textural habit; the latter texture is shown in the mottled ap- 

 pearance brought out by weathering, which in later stages develops into 

 a deeply pitted condition of the surfaces. The examination of the local- 

 ity does not show any relationship between position in the series and 

 textural conditions. Localh^ a porphyroidal habit is seen, confined in 

 its distribution chiefly to the am3^gdaloidal phases of the rock. 



The more conspicuous character of the diabases is a prevailingly dark 

 color, which is in some flows a dark green to black ; in others a dark 

 l)rown. From these as foundation colors green obtains when the altera- 

 tion has produced a considerable proportion of chlorite, and brown when 

 a ferric oxide has formed. 



Gradually and quite steadih' upward from the bottom of each flow 

 the texture becomes coarser. Texture is not always the same ; it varies 

 with the thickness of the flow, and thus becomes an item of evidence in 

 working out the history of the units of the series. Locally, variations 

 were noted. A concentric weathering is one variation. It occurs in 

 thick and thin flows alike, but seems to be confined to the granular 

 textured rock (see figure 2). In two or three other flows of a decidedly 

 ophitic habit, banding is seen. Its origin is obscure, since it seems to 

 run at right angles to the direction of bedding. The foregoing structures 

 seem to occur without regard to the nature of the original or existing 

 mineral constituents. 



It is in the upper vesicular portions that the greatest changes are seen. 

 These consist in the deposition of new minerals within the vesicles of 

 the amygdaloid ; in the form of pseudo-amygdaloid through the replace- 



* The Crystal Falls iron-bearing district of Michigan. Monograph xxxvi, U. S. Geol. Survey, 

 1899, p. 141. 



