PETKOGRAPHIC GEOLOGY AND DESCRIPTIONS. 123 



Zirkelyte. Diabase. Tuff.] 



No. 22. ZIRKELYTE. ( Amygdatoidal. ) 



Duluth. Overlies No. 21. Extends about 300 feet along the lake shore. 

 Ref. Annual Report, ix, page 14. 



Meg. A dark gray, aphanitic rock confusedly mottled with black and yellowish 

 areas, the latter apparently containing considerable chlorite. The rock thus seems 

 to be an amygdaloid. 



Mic. The main part of the section contains many minute feldspars, mostly 

 lath-shaped, in and between which is a confused fine-grained aggregate of magnetite, 

 chlorite, epidote, leucoxene, and apparently a little quartz and calcite. The rock is much 

 altered, and it is thought to have originally contained some glassy material, although 

 this is not certain. Scattered through the section are amygdules, or rather pseud- 

 amygdules, which contain quartz, chlorite, epidote and calcite. Much of the quartz 

 shows distinct undulatory extinction, thus indicating that the rock has been subjected 

 to pressure or strain since the quartz was deposited in the cavities. 



One section. 



Age. Cabotian. u. s. G. 



No. 23. DIABASE (with olivine). 



Duluth. From a dike which cuts No. 22, fifteen feet wide, running north and south. 



Compare No. 8C. 



Ref. Annual Report, ix, page 14. 



Meg. A fine grained, brownish black rock. 



Mic. Feldspar, not much stained, appears in lath-shaped microlites, and in the 

 angular interspaces is seen the remain ing stage of the augites which are considerably, 

 though not entirely, altered to chloritic and ferruginous products. 



Olivine, in the same manner, appears in the form of greenish spots, which are 

 accompanied by magnetite, but these differ from those resulting from augite, in 

 the fact that they crowd on the shapes of the feldspars, having preceded the feldspars 

 in date of formation. 



One section. 



Age. Probably Manitou. N. H. w. 



No. 24. TUFF. 



Duluth. Next east of the dike represented by No. 23, with laminations of green. It is coarser than No. 

 17, dips 35 to 40 easterly, and extends fifty feet along the beach. 



Ref. Annual Report, ix, pages 14, 15; American Geologist, vol. xviii, pages 211-213. 



Meg. Apparently a fragmental rock of a brown, granular appearance, but without 

 evident silica grains. 



Mic. The slide is made up of irregularly shaped areas of lighter and darker 

 rock, the latter being apparently embraced in the former. The dark areas are fre- 

 quently rounded but not wholly. Some are elongated and more rough, and some of 

 them are themselves vesicular. The minerals in these dark areas are magnetite, 



