PETROGEAPHIC GEOLOGY AND DESCRIPTIONS. 131 



Diabase.] 



layer twelve feet thick; much decayed; fine grained; evidently a phase of the same 

 rock as No. 34. 



Mic. The feldspar is reddened, lath-shaped, and idiomorphically distributed 

 amongst the other grains, which, however, are none of them in their original condi- 

 tion. They consist of magnetite, hematite, chlorite, glass ( ?), epidote, leucoxene. In 

 strong polarized light (the same is less evident in natural light) and high powers the 

 whole section glitters with bright specks, either sericitic or chloritic, or with fibres 

 which appear to be serpentinic. This effect is hightened by lowering the polarizer. 

 Even the opaque grains of magnetite are thus seen to be impure. They are more or 

 less involved in this iridescence, and enclose minute parts of the minerals for which 

 they are substituted. This is another evidence of their secondary nature. Most, if 

 not all, of the magnetite in the section is of this character. The only evidence of 

 original titaniferous magnetite, or of ilmenite, consists in occasional gray sub-trans- 

 lucent grains which are apparently kuco.n'nc. 



One section. 



Age. Cabotian. N. H. w. 



No. 36. DIABASE (?) 



Duluth. West side of Chester creek. 

 Ref. Annual Report, ix, page 16. 



Mey. Hard, gray, or brownish-gray, fine-grained, and fresher condition of the 

 eruptives at the mouth of Chester creek, a basic trap with alternating and irregular 

 belts of amygdaloid. 



Mic. The feldspars are very fine and lath-shaped, lying ophitically amongst the 

 more or less altered pyroxenes and the remnants of the magma. 



The pyroxene occasionally shows its bright polarization colors. The sections 

 show much of the light-green, amorphous substance supposed to result from residuum 

 from the magma. 



Two sections. 



Age. Cabotian. . N. H. w. 



No. 36A. DIABASE (?) (Amygdaloidal.) 



Duluth. Amygdaloidal porous condition of No. 36, from the upper portion of the bluff immediately west 

 of the mouth of Chester creek. This porous condition occurs in layers or belts rn No. 36. 

 Ref. Annual Report, ix, page 16. 



Meg. An aphanitic brown to reddish rock in which are numerous cavities of 

 all sizes up to those a quarter of an inch in diameter. These cavities sometimes 

 contain epidote, and this mineral also permeates almost the whole rock. 



Mic. The slide is made up of numerous feldspar microliths in a groundmass 

 which is composed of magnetite, in large amount, and an almost colorless isotropic 

 substance. Under a high power this isotropic substance is seen to be greenish, 



