Laurentian Mountains, a range of British North America, extending in its general direction from Labrador in a curve around the S. and W. shores of Hudson bay to the Arctic ocean, upward of 3,000 m. In its E. portion it forms the watershed separating the tributaries of the St. Lawrence river from those of Hudson bay. Beyond the basin of the St. Lawrence it is traversed by two affluents of the bay, Nelson and Churchill rivers; and still further N. it divides for 800 m. the tributaries of Mackenzie river from the streams flowing into Hudson bay. Its general elevation is from 1,500 to 1,600 ft., with some peaks about the Saguenay river of 4,000 ft. It is mostly well wooded with pine or spruce on the summits, and hard wood on the lower elevations and in the valleys. A remarkable feature of the range is the immense number of ponds and lakes, expansions of the streams, that dot its surface. The principal break in its general direction is on the Ottawa river, above Ottawa, where it crosses that stream, sweeps round S. to the Thousand islands near the exit of the St. Lawrence from Lake Ontario, then bends N. W. to the S. extremity of Georgian bay, and continues along the N. E. and N. shores of Lakes Huron and Superior, resuming its general course.

The rocks of this range are sedimentary strata, which have become highly crystalline, and are the most ancient known on the American continent. The Canadian-geologists have given to this formation the name of the Laurentian system.