752 PRE- CA MBRIA N NOR TH A M ERICA N L ITER A TURE 



layers of sedimentary rocks. This structure usually dips to the south. 

 It does not depend upon the differentiation of the mineral compo- 

 nents of the rocks, but seemingly is due to secondary causes which 

 acted upon the rock after it had solidified. This sheeted structure is 

 a common phenomenon along the northern limits of the mass. The 

 gabbro has also a banded structure due to the parallel arrangement of 

 the mineral constituents. The bands are not regularly arranged, 

 appearing and disappearing in a manner which shows them to be not 

 independent of the secondary causes. This structure is present to a 

 marked degree in the central portion of the gabbro. 



Large feldspar masses occur in the gabbro in the southeastern 

 parts of T. 6i N., Rs. lo W. and ii W. The mass in the latter town- 

 ship has a marked banding. The line of division between the feld- 

 spar masses and the normal rock is sharp in the field and in the hand 

 specimen. Both are, however, regarded as differentiations from the 

 same magma. 



In the southern part of T. 62 N., R. 10 W., the eastern part of T. 

 61 N., R. II W., the greater part of T. 61 N., R. 10 W., and in 

 adjacent townships is a considerable area of dark, reddish-colored 

 olivine-gabbro or troctolyte, which has both a sheeted and banded 

 appearance. This rock and the normal gabbro have not been seen in 

 contact, but wherever they closely approach each other, often within a 

 few feet, both preserve their characteristic structure, and there is no 

 sign of the transition of the one into the other. The olivine rock 

 appears to be above the ordinary gabbro. 



Hubbard' gives two geological cross-sections of the Keweenawan 

 series in the vicinity of the Calumet and Hecla and the Tamarack 

 mines. The strata here consist of interstratified traps, amygdaloids, 

 sandstones, and conglomerates. Deep in the series there is less amyg- 

 daloid, and it is suggested that the amygdaloids are largely pseudo- 

 amygdaloidal, their development being dependent upon sub-surface 

 weathering. It is found that the conglomerates approach each other 

 in passing from the north toward the south, due to the thinning of 

 the igneous beds. The Eastern sandstone, somewhat remote from the 

 line of junction with the Keweenawan series, has at places a dip toward 

 the traps as high as 10° or 12°. At Lake Linden this formation is 



'Two New Geological Cross-sections of Keweenaw Point, by L. L. Hubbard. 

 Proceedings of the L. S. Mining Inst., Vol. II, 1894, pp. 79-96. 



