PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 241 



Basalt.] 



and are overlain by a bed of trap undistinguishable from the trap that occurs 

 frequently along here, and underlain by the next. 



From Temperance river to this place, the rocks consist of the same series, viz.: 

 a series of volcanic debris, making sandstones, resulting from the disintegration of 

 lava flows, appearing as conglomerate and as amygdaloid, with occasional sheets of 

 firmer trap. The amygdaloidal minerals are largely laumontite and calcite, but these 

 are also in the sandstone in the form of small veins and otherwise. 



No section. 



Age. Potsdam. N. H. w. 



No. 176. BASALT (u-ith olivine). 



Six miles east of Temperance river. Underlies a fragmental series of sandstone, shale and conglomerate. 

 Ref. Annual Report, ix, pages 47, 48. 



Meg. A tough, heavy, thin bedded rock, having a red" mineral (heulandite?) 

 separating its frequent joints, so as to appear blood-red on approach, or spotted 

 blood-red. Its interior color is dark brown or black, and it is seamed with calcite 

 and laumontite, the second including the former as between the walls of a vein, the 

 veins being rarely more than one-fourth of an inch in thickness. It is scantily 

 pseudamygdaloidal with the same minerals; twenty-two feet thick; resembles the 

 Two Harbor rock (No. 117). 



Mic. The groundmass is fine, and dark colored between the nicols, the feldspar 

 microliths lying in the midst of a minutely granular mass. The feldspar had two 

 periods of generation. The larger crystals have an extinction angle that ranges low, 

 approaching andesine. They are of irregular forms, the separate twins not having 

 uniform lengths nor widths. In a section which is perpendicular to an optic axis 

 extinction occurs at from 6 to 10 in the lamella showing the optic axis, and the 

 optic plane forms an angle of 22 with the brachypinacoid. This section is also 

 perpendicular to the ''plane of symmetry," as the term is used by Michel Levy, i. e., 

 it is in the zone perpendicular to the edge 001 :010. This is shown by its having four 

 positions in a single rotation, at which the twinning bands are equally illuminated, 

 and cannot be distinguished. Only anorthite can present these optic characters 

 combined.* There are others of the larger crystals of the plagioclase that do not 

 seem to be anorthite. One such cut perpendicular to the bisectrix n g has extinction 

 at about 16, which according to Fouquef can take place in a labradorite near 

 andesine, or in an oligoclase near albite. If anorthite was crystallized from the 

 magma, simultaneously with this, it is not likely that this differs from anorthite so 

 widely as to be an oligoclase-albite. It is therefore more likely to be labradorite- 

 andesine. 



* Etude sur la determination des feldspaths, plate VII. 



t Bulletin de la Sociiti de Mineralogie de France, vol. xvii, p. 428. 



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