GEOLOGICAL AND NATURAL HISTORY SURVEYS. 179 



day, slates, slaty hornblende rock, and (]nartz rock — the latter con- 

 stituting by far the largest proportion of the whole group " (p. 17). 

 These rocks stretch into the interior in a southwesterly direction. 

 The average width of the belt does not exceed 6 or 8 miles. The 

 cleavage of the rocks is usually north or N. 10' W., with an angle 

 of about 80°. The dip of the strata is south or southwest. In the 

 talcose slate, as we approach the granitic region occurs a rock de- 

 nominated serpentine, but bearing close resemblance to greenstone, 

 being essentially composed of granular feldspar and hornblende, 

 with which serpentine is intimately blended. The riictaniorphic 

 rocks are occasionally traversed by trap dikes. 



4. Conglomerate. — This is not known east of the commencement of 

 the trap group, " nor has it been noticed resting upon any of either 

 the primary or metamorphic rocks, but is invariabl}^ seen resting 

 opon the trap rocks" (p. 17). It flanks the trap on its northerly 

 side, from the extremity of Keweenaw Point to the head of Lake 

 Superior. On He Ivoj'ale a similar rock rests upon the trap, facing 

 southeast. It forms hills on the south shore 300 to 500 feet high. Its 

 thickness increases westward, being 5,2G0 feet a little cast of j^Iontreal 

 River, and 1,000 feet at its eastern extremity. But the thickness is 

 extremely variable. It may, " without doubt, be considered as a 

 trap tuff which was gradually accumulated around the several coni- 

 cal knobs of trap, during their gradual elevation" (p. 33). The 

 constituent pebbles are rounded masses of greenstone and amygda- 

 loidal trap, with scarcely a pebble of any different rock. They vary 

 in size from that of a pea to several pounds in weight, but average 

 1^ to 2 inches in diameter. They are generally united by a mixed 

 calcareous and argillaceous cement, more or less colored by iron. 

 It is imperfectly stratified, even in masses of immense thickness. 

 Its dip, on the south shore, is northwest at angles of 30° to 85°. 

 On He Royale the dip is reversed. It is frequently traversed by 

 dikes of trap having a thickness of 50 to 60 feet, or even several 

 hundred feet; and these are usually parallel to the plane of strati- 

 fication. But there are also veins of more recent origin, and these 

 are metalliferous. 



5. Mixed conglomerate and sandrock. — This is an alternating 

 series of coarse conglomerates and red sandstones, resting conform- 

 ably on \\\<i conglomerates just noticed. They are strictly a member 

 of the conglomerate, and only separated for convenience of descrip- 

 tion. Tlie formation is 4.200 feet thick on the flanks of the Por- 

 cupine Mountains and wedges out in both directions, disappear- 

 ing near the extremity of Kew^eenaw Point. The dip is regularly 

 north and nortliwcst into the bed of the lake. It was not noticed 

 on the north shore nor on He Royale. The sandstone portions are 



