440 THE GEOLOGY OP MINNESOTA. 



[Marble. 



uted to the nearness of the great vein on which the mine was operated, and which 

 is also corroborated by the small calcite veins which cross the specimen. 



Quartz forms micropegmatyte in the feldspars, and also isolated, independent 

 grains. Sericite( ? ) and other gray substances are distributed through the feldspars, 

 rendering them so obscure that their albite twinning is almost destroyed. Biotite is 

 surrounded occasionally by chlorite, which also spreads more widely, especially in 

 some of the feldspars. Olivine is not distinguishable, and the any tie, which is detected 

 by its ophitic relation to the feldspars, is changed to a dirty, fibrous, ferruginous 

 substance that in some places may approach hornblende, but in general is greenish, 

 and related to some chlorite. Apatite, magnetite and a small amount of titanite also 

 appear, the last forming groups of tapering, lath-shaped crystals with high refrac- 

 tion and high double refraction, having much the color and irregularity of cleavage 

 seen in ferruginous olivine. 



One section. 



Age. Cabotian dike in the Animikie. 



Bemark. The condition of this rock, at 720 feet below the surface, cannot be 

 attributed to ordinary weathering. It is the most obvious inference that it is due in 

 some way to the presence of the great dike which here is known to cross the vein 

 and the country rock. Whether it can be attributed to the greater ease with which 

 surface waters could enter along the walls of such a fissure, or to the heat imparted 

 to the walls, resulting in an action and retroaction of the walls on the dike, is appar- 

 ently the only question that need be considered. The mineral contents of the vein 

 seem to show that the mineral solutions came from below rather than from above, 

 and the presence of considerable amounts of graphite indicate a high temperature. 

 It seems, therefore, that the condition of the igneous rock is due to a reaction from 

 the country rock. In that respect this dike illustrates what has been inferred from 

 an examination of several dikes on Pigeon point and elsewhere, viz., the entrance of 

 quartz cotemporary with the alteration of all the minerals of the dike during the 

 process of cooling. N. H. w. 



No. 591. MARBLE. 



Silver Islet. From the vein worked for silver. 

 Ref. Annual Report, x, page 55. 



Meg. This is massive calcite, varying in color from gray-white to flesh-red-white. 

 In the gray portion is visible some galena, and in the flesh-colored some pyrite. 

 Mic. There is a little quartz, in angular grains, isolated in the mass. 

 One (thick) section. 

 Age. Vein in the Animikie. x. n. w. 



