126 THE GEOLOGY OP MINNESOTA. 



[Diabase. Basalt. 



No. 27. DIABASE (with olivine). 



Duluth. A dike cutting No. 26. This dike changes its direction twice. It leaves the lake in a direction 

 east and west, but on ascending the rocky bluff it immediately changes to W. 15 S. It runs so about eight feet 

 and shifts again to nearly west. 



Ref. Annual Report, ix, page 15. 







Meg. Very fine grained and dark; not porphyritic. 



.]//<. Fine feldspathic microlites lie in the midst 'of the usual ferro-magnesian 

 and magnetic minerals. This rock differs in one particular from many of the asso- 

 ciated and similar rocks already described. A considerable amount of any He still 

 remains. This is colorless and shows alteration to greenish to colorless grains, which 

 often have borders of mai/in/tite. These grains are fibrous in structure, have a low 

 extinction angle, and polarize in rather bright colors. They are referred to Jiornblctn/c. 

 In No. 26 there is no augite, but the hornblende grains are very "abundant and are 

 similar to those in No. 27. 



One section. 



Age. Probably Manitou. N. H. w. AND u. s. G. 



No. 28. DIABASE (iritli oliciitr). 



Duluth. Next east of No. 27. 

 Ref. Annual Report, ix, page 15. 



Meg. Fine grained but scantily porphyritic with the earlier plagioclases. This 

 rock is quite similar to No. 27, but when collected it was considered the continua- 

 tion of the rock on the easterly side of the dike represented by No. 26. It has an 

 evident bedded dip, extends 300 feet and passes, on the east, under No. 29. 



Mic. This rock differs from No. 26 in wanting the irregular red areas, but it 

 shows some scattered red f/'/tlxpars. At the same time it very closely resembles No. 27. 



Age. Cabotian. 



Remarks. Nos. 26, 27 and 28 are lithologically closely allied, but structurally 

 they are widely separated. No. 27 is a dike which cuts the rock Nos. 26 and 28, these 

 two being from opposite sides of the dike. It is only on the ground of these struc- 

 tural relations that the dike is classed as probably Manitou and the other as probably 



Cabotian. N. H. w. 



No. 29. BASALT. (Amygdaloidal.) 



Duluth. Apparently the amygdaloidal upper portion of No. 28; about five feet thick. 

 Ref. Annual Report, ix, page 15. 



This rock is much altered to fpldofr, and it crumbles in the weather. Micro- 

 scopically it differs none from several that have been described. 



Age. Cabotian. N. H. w. 



