PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 281 



Conglomerate.] 



substance, whose nature it is difficult to determine. There are other sub-isotropic 

 grains, filled with impurities, some of them having a poikilitic quartz background, 

 and some that are derived from feldspathic grains, now much altered. The only 

 plausible interpretation of the isotropic grains, from a petrographic point of view, 

 which presents itself, is to assign them to an original volcanic glass, which may have 

 existed in considerable quantity in the vicinity where this conglomerate was being 

 formed. Structurally, however, there is no warrant for such a supposition, and it 

 seems necessary to refer these isotropic and sub-isotropic grains to alteration from 

 debris derived from some crystalline rock of older date. (See Part I; compare 

 No. 2069.) 



Two sections. 



Age. Basal conglomerate of the Potsdam. 



Four additional sections, from pebbles of this conglomerate, were made subse- 

 quently by Marchand. Of these, two were of the "red rock" pebbles. They show a 

 rock consisting largely of qinoiz, but between crossed nicols so dark that it is neces- 

 sary to allow the existence of considerable isotropic matter. The quartz is in small, 

 irregular grains, sometimes clear and sizable, but for the most part closely inter- 

 grown and overlapping, with tortuous outlines. The pebble also embraces triclinic 

 feldspar fragments, and calcite spreads abundantly through some parts of it. There 

 are also other small, brightly-colored and lamello-fibrous crystalliths which can 

 hardly be named specifically, but may be of muscovite. The aspect is that of a com- 

 posite, siliceous rock, hardened and partially recrystallized by heat, but not wholly 

 fused. Surrounding this pebble are other, smaller, clastic grains and composite 

 pebbles, some of them being apparently from a rock that would bear the name of 

 quartz-porphyry, with bipyramidal quartzes whose orientation controls the poiki- 

 litic quartz that surrounds them and embraces more or less colored and isotropic 

 matter. 



Of two other sections made from pebbles from this conglomerate, one is of a 

 granular quartzyte resembling much the rock No. 1838. Mingled with fine, rounded 

 quartz pebbles are some that are clouded with reddish impurities. These last have 

 regular extinctions, and appear to have resulted from a silicification of an older rhyo- 

 lyte, but on testing them in convergent light they exhibit indistinctly, when cut 

 favorably, the single dark bar of a biaxial mineral which rotates with the stage, but 

 in reverse order. They are therefore clastic grains of orthodase. In the same pebble 

 are other minerals. Epidofe shows by its high refraction and double refraction. It 

 is rather abundant. A dichroic, fibrous, greenish to brownish mineral, somewhat 

 resembling hornblende, is probably clinoclilore, as its double refraction, though in 

 the colors of the first order, is but little above that of quartz cut in the same thick- 



