124 THE GEOLOGY OP MINNESOTA. 



[Zirkelyte. Diabase. 



epidote, calcite and an isotropic substance which is greenish and similar to that which 

 frequently follows an alteration of olivine or augite. These areas are darker, appar- 

 ently, because of the greater proportion of the magnetite. The matrix which 

 embraces these darker areas consists of about the same secondary minerals, with 

 also much devitrified glass, but having less of the greenish isotropic substance and 

 less of the magnetite. Also, the opaque substance has more frequently the color of 

 hematite than of magnetite. Quartz is not wanting, but is very scarce. 



Two sections examined. 



Age. Cabotian. 



Remarks. This rock, when collected, like several others obtained in the imme- 

 diate vicinity, was considered a shale belonging to the sedimentary series. It is not 

 probably wholly, nor perhaps chiefly, due to erosion of other rocks, but its general 

 stratiform structure points to the action of oceanic forces in giving distribution to a 

 fine ash, in which were mingled also larger masses of a vesicular rock of the same 

 nature as the ash, but probably of the origin of volcanic lapilli. N. H. w. 



No. 25. ZIRKELYTE. (Amygdaloidal. ) 



Duluth. Occurs twenty feet east of No. 24 and extends 100 feet. 

 Ref. Annual Report, ix, page 15. 



Meg, A brownish aphanitic rock in which are many amygdules, some of which 

 are half an inch across. The amygdules contain much epidote; especially are the 

 outer layers of the amygdules of epidote. Calcite, sometimes in crystals of consid- 

 erable size, also occurs, as does a soft, greenish, clay-like material. 



Mic. The section is too thick for careful study. It is composed almost entirely 

 of quartz and an opaque greenish yellow substance. The section was evidently cut 

 from one of the amygdules. 



One section. 



Age. Cabotian. u. s. G. 



No. 26. DIABASE (with olivine, altered). 



Duluth. A rocky point near the old Fishery. 

 Ref. Annual Report, ix, page 15. 



Meg. The rock is aphanitic and almost black. Its most noticeable feature is 

 the presence of numerous red areas which at first appear like porphyritic feldspar 

 crystals. But on closer inspection these areas are seen generally not to possess 

 crystal outlines; they are in fact very irregular in outline and are not rounded like 

 amygdules. The red material exhibits no cleavage surfaces; it is easily scratched 

 with a knife, and effervesces with cold nitric acid. There are also other less sharply 

 defined yellow areas (epidote), and the lens shows that these yellow areas, often of 

 minute size, extend all through the rock. Two rounded areas of calcite are seen in 



