PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 133 



Graywacke. Diaba&e.] 



The pyroxene in the body of the rock is ophitic and tolerably fresh. A vein or 

 fissure of later date crosses the section, and this is filled with chlorite scales standing 

 vertically on the sides of the fissure. This chlorite, in some instances, spreads more 

 widely, affecting chiefly the pyroxene. In many cases this chlorite is in leaves, 

 favorably cut, and shows a strong pleochroism. 



Magnetite is in angular masses, as if it had taken the position of some mineral, 

 or of a residuum of the magma, since the consolidation of the feldspars. It is also 

 in fine particles in the altered pyroxenes. 



The good preservation of this rock, in contrast with the sheets of eruptive rock 

 about Chester creek, the latter cut by the former, seems to prove a shorter period of 

 exposure for the dike or better protection from the atmosphere. 



One section examined. 



Age. Manitou. N. H. w. 



No. 39. GRAYWACKE. (Breccia.) 



Duluth. On the east side of the dike, No. 38. Extends thirty -five feet, and dips east. 

 Ref. Annual Report, ix, page 16. 



Meg. The most of the hand specimen is composed of a fine-grained siliceous 

 rock, which might be called graywacke. It has pyrite crystals disseminated 

 through it. It has on one side, and was apparently surrounded when in situ by a 

 very different kind of rock. -The appearance is as if a gray siliceous breccia had a 

 cement of a basic eruptive. 



Mic. The rock is made up of fine angular and sub-rounded fragments of quartz 

 and plagioclase, and of devitrified glass, with a few scattering grains of pyrite, and 

 a few larger areas of secondary quartz. Except in its much greater content of quartz 

 this included rock is similar to some already mentioned, viz.: Nos. 7B, 8B, 21 and 30. 

 Its fragmental manner of occurrence in the lava is also similar. 



Two sections examined. 

 Age. Inclusion in Cabotian eruptives. N. H. w. 



No. 40. DIABASE (ivith olivine). 



Duluth. East of Chester creek, at the lake shore, about forty feet east of the dike, No. 38. Hard, firm 

 beds, in thin contorted layers, varying in durability and in color, some greenish and some brown, some nearly 

 black. The whole is fine-grained, compact, of a dark grayish or brownish color. 



Ref. Annual Report, ix, pages 16, 17. 



Meg. Compact, rather fine grained, occasionally with a porphyritic feldspar 

 crystal, and other colored spots. Undistinguishable from numerous others that out- 

 crop between this point and Rice's point. 



