LAURENTIAN SYSTEM. 19 



extends north-westerly a short distance north of Lake Huron, and bordering upon 

 Lake Superior, a great part of its length, it appears at Lake of the Woods, north 

 of Rainy River, though an arm extends south of Lake Superior into Michigan and 

 Wisconsin. The western boundary of this great area extends from Lake of the 

 Woods in a sinuous northerly direction among the lakes, and following the highlands 

 that divide the waters which flow into Hudson's Bay from those flowing in other 

 directions, to the Arctic Ocean. There are some patches within this general outline 

 covered with Taconic rocks, or those of Post-pliocene age. 



§ 33. The arm of this great exposure, which appears in the Upper Peninsula 

 of Michigan, has an area of about 1,839 square miles, consisting of several tracts, 

 one of which touches Lake Superior west of Marquette. The rocks are chiefly 

 granite, gneiss, syenite, and crystalline limestone, which thus far have afforded no 

 useful minerals. The surface area in Wisconsin is somewhat greater. Other ex- 

 posures in the United States are confined to irregular areas in the mountain regions. 

 North Carolina exposes about 20,000 square miles, or nearly half the State. One 

 belt from twenty to twenty-five miles wide, crosses the northern part of the sub- 

 eastern section of the State upon which the capital is situated. It extends north- 

 ward into Virginia, and southward beyond Cape Fear River. It consists generally 

 of gneiss, which passes into granite or mica schist. Another belt extends from the 

 southern border of the State at Catawba River in a north-east direction, almost to 

 the Virginia line near Roxboro, and reappears eight or ten miles to the eastward 

 and crosses the northern border about midway of Granville County. There is 

 another small area in the southern part of Orange County. Limited areas are found 

 in Georgia, Virginia, Pennsylvania, New Jersey, Vermont, New Hampshire, and 

 at other places in the Appalachian chain. A small area occurs in Missouri near 

 Iron Mountain, and another in Arkansas. In the Rocky Mountain region there are 

 many exposures, some of which are quite large. They generally trend in the di- 

 rection of the mountain chain, and are found in Mexico, New Mexico, Arizona, 

 Nevada, Utah, Colorado, Idaho, Nebraska, Wyoming, and Montana. 



§ 34. A section taken by Logan in the region where he studied the rocks, is 

 as follows: 



1. Orthoclase gneiss, composing Trembling Mountain, 5,000 feet. 



2. Crystalline limestone of Trembling Lake, 1,500 " 



3. Orthoclase gneiss, 4,000 " 



4. Crystalline limestone of Great Beaver and Green Lakes, with inter- 



stratified garnetiferous rock and hornblendic orthoclase gneiss, 2,500 " 



5. Orthoclase gneiss, garnetiferous gneiss and quartzite below the 



Grenville limestone, 3,500 " 



6. Crystalline limestone of Grenville, with interstratified gneiss, . . 750 " 



7. Orthoclase gneiss, 1,580 " 



8. Proctor's Lake limestone, 20 " 



9. Orthoclase gneiss, passing gradually into Anorthosite between 



Proctor's Lake and Morin band, , 3,400 " 



10. Anorthosite above the Morin band, ' 10,000 " 



Total, 32,250 feet. 



This is about the thickness in New Hampshire, and not equal to the estimated 

 thickness in Wisconsin. Enough is known, however, to show this section of Logan's 

 by no means represents the total thickness in Canada. The better opinion seems to 

 be that the Laurentian series has a thickness in Canada of more than 40,000 feet. 



