20 MICHIGAN SURVEY, 1905. 



by Irving ('83), but little attempt has been made to correlate it with the 

 topography, in which connection it chiefly concerns the distribution of animals 

 and plants. 



The rocks of this part of the Northern Peninsula belong to what is known 

 as the Keweenawan Series, and may in a rough way be divided into three 

 classes, — the basic or trap rocks, the porphyries or acid rocks, and the de- 

 trital rocks. 



The basic rocks make up the greater part of the whole series. They are 

 finely crystalhne rocks formed by the slow cooling of molten lavas contain- 

 ing 45 per cent to 50 per cent of sihca, and consist of a lower compact portion 

 grading upward into a V£sicular or amygdaloidal portion interstratified 

 with bands of unaltered red sandstone and conglomerate which become 

 more numerous near the top. These basic rocks are true eruptives and 

 constitute successive lava overflows from fissure eruptions (Irving, '83, p. 

 139). 



The acid rocks occur between layers of basic rocks and are in part, like 

 the basic rocks, true eruptives. They are composed qi materials which fuse 

 with difficulty, owing to the presence of silica, and which consequently 

 cool rapidly into rocks composed of large crystals embedded in a glass}^ or 

 porcelain like matrix. 



The detrital members of the series consist of sandstones, conglomerates 

 and shales formed from acid, or less prominently basic, rocks. 



The Keweenawan Series is divided by Irving into two divisions, the Upper 

 and Lower Keweenawan. The Upper Division consists wholly of detrital 

 material, Avhile the Lower consists of alternating flows of lavas and sediments. 

 Most of the Upper Division lies under Lake Superior, but it also underlies 

 the broad plain between Portage Lake and the Porcupine Mountains. This 

 plain is interrupted by the Porcupine Mountains but continues again to the 

 south. 



The Keweenawan rocks border nearly the entire area of the Lake Superior 

 basin. Besides forming the larger part of Keweenaw Point and constitut- 

 ing the Michigan shore to the Montreal river, they occur in northern Wis- 

 consin, eastern and northeastern Minnesota and occupy a great area about 

 Lake Nipigon. The outcropping trap rocks in Michigan occur as a range 

 extending from the end of Keweenaw Point southwestward into Wisconsin. 

 From this range the detrital sandstones and conglomerates dip away toward 

 the lake. In the vicinity of the Porcupine Mountains, an outcrop of trap, 

 owing to a fold, leaves the main range and swings away toward the lake, 

 in T. 50 N., R. 44 W., and returning joins the main range in T. 49 N., R. 45 

 W.; within this fold occur the rugged ranges of the Porcupine Mountains. 



3. GENERAL TOPOGRAPHY. 



The mountains rise from the south shore of Lake Superior, Fig. 1, in Town- 

 ship 51 North, Ranges 42, and 44 West, as a convex ridge parallel to the lake 

 shore. Back of this ridge they extend to the south as steep ridges and 

 knobs in T. 50 N., Rs. 42, 43, 44 and 45 W., and T. 49 N. Rs. 43, 44, and 45 

 W. The whole system comprises three fairly distinct parallel ridges that 

 nowhere attain an alpine height; the highest point, according to the charts 

 of the United States Lake Survey, being 1421 feet above the level of Lake 

 Superior or 2022.19 feet above sea level. Unfortunately the territory 

 covered by the topographic map of the Michigan Geological Survey does 

 not embrace this section, so this record has not been verified. The altitudes 



