PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 159 



Amygdaloid. Diabase.] 



essentially of quartz, are secondary after glass. They are rounded, and are dimmed 

 by many inclusions of opaque substance. Still others are angular plagioclase frag- 

 ments, their twinning being perfectly apparent by reason of the banded extinctions. 

 These are embraced, over large areas sometimes, by calcite which like the Fontain- 

 bleau crystals presents a single orientation. 



One section. 



Age. Cabotian. 



Remark. This rock may be compared with Nos. 17, 24 and 30. It occurs as 

 angular inclusions in a vesicular lava, No. 72A. The rounded outlines and the 

 uniformity of size of the most of the grains seem to be due to beach friction, for it 

 is hardly possible that such forms can be produced by volcanic explosive action. 

 The grains of silicified glass are comparable with the matrix of some of the apor- 

 hyolytes of the region. N. H. w. 



No. 72A. AMYGDALOID. (Calcitic.) 



Mouth of Lester river, east of Duluth. The cementing material of the foregoing breccia. 

 Ref. Annual Report, ix, page 22. 



Meg. A dull, much changed, fine-grained rock, containing amygdaloidal cavities 

 mostly occupied by calcite. 



Mic. Minute trachytic feldspars are distributed throughout the matrix. The 

 ferro-magnesian silicates are wholly changed to serpentinous or chloritic elements, 

 which also fill certain amygdaloidal spaces. No calcite amygdules appear in the 

 section. The chloritic amygdules are blue between crossed nicols, but sometimes 

 have a green border surrounding the blue. 



Age. Cabotian. 



Remark. It is a common feature to find these clastic masses in the midst of 

 the vesicular lavas of the region. It is as yet unexplainabie. Such association may 

 have resulted from the rupture of old clastic strata in the ejection and flow of the 

 lava, or from cotemporary erosion and sedimentary action of the ocean's waters on 

 the lavas of the same or nearly the same date as the amygdaloids in which they are 



found. N. H. w. 



No. 73. DIABASE. (Finegrained.) 



East side of the mouth of Lester river. Becomes brown along some of the joints, and in some large areas. 

 Extends one and one-half miles, with a line of low exposure. Runs under No. 74. 

 Ref. Annual Report, ix, page 22. 



Meg. A fine-grained, compact, dark -gray rock. Small feldspars ace the only 

 crystals that can be distinguished. 



Mir. The section is composed of small lath-shaped plagioclase in a background 

 of aiigiie, magnetite, plagioclase and some secondary products. In structure this 

 rock differs some from the usual diabases. The lath-shaped plagioclases are not all 



