PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 173 



Diabase.] 



No. 98. DIABASE. ( Amygdaloidal. ) 



West side of Agate bay; underlies No. 97. 



Ref. Annual Report, ix, pages 26, 27. Annual Report, x, page 37. 



Meg. A dark greenish rock, very fine-grained, with a few rather large, black- 

 green amygdules of chlorite. The same mineral occurs in cracks in the rock. 



Mic. Minute lath-shaped plagioclase in a background of augite grains, among 

 which is feldspathic material and some interstitial matter which has no or almost 

 no effect on polarized light, and which probably represents original glassy material. 

 The section shows a few small amygdules of radiating chlorite, and one foreign 

 fragment which is composed of lath-shaped feldspars, much larger than those of the 

 rest of the section, in a green mass of alteration products. 



One section. 



Age. Cabotian. u. s. G. 



No. 99. DIABASE. (Amygdaloidal.) 



West side of Agate bay; underlies No. 98. 



Ref. Annual Report, ix, page 27. Annual Report, x, page 37. 



Meg. Almost exactly exactly identical with Nos. 95 and 97, which see. 

 No section. 



Age. Cabotian. u. s. G. 



No. 100. DIABASE. (Fine.) 



PLATE I, FIGURE 1. 



West side of Agate bay; underlies No. 99. 



Ref. Annual Report, ix, page 27. Annual Report, x, page 37. 



Meg. A dark gray, very fine-grained, compact rock, having a few amygdules 

 of chalcedony. 



Mic. Small lath-shaped feldspars in a groundmass made up mostly of large 

 augite plates, a fine example of lustre-mottling on a small scale. Between the augite 

 plates is frequently a greenish to brownish material, almost opaque and containing 

 abundant iron ore. In polarized light and under a high power, some of this material 

 is seen to be minutely granular in structure. This feature is quite common in some 

 of the fine-grained diabases of the Keweenawan of the south shore of lake Superior, 

 and it is thought that this greenish to brownish material represents part of the 

 magma that was imperfectly crystallized or was even glassy, now much altered. 

 Compare especially plate ix, of Irving's " Copper-bearing Kocks." 



One section. 



Age. Cabotian. 



Remark. This specimen well illustrates the lustre-mottling and the interstitial 

 " glassy " material. u. s. G. 



