Geologically, the Laurentian Plateau forms part of the vast "Canadian Shield" which covers more than half the area of the Dominion. The rocks are for the most part Precambrian and of igneous origin, but enfolded with them are crystalline limestones and other altered sedimentaries of Precambrian age. The pre dominant rocks, however, are granites and granite-gneisses, rep resenting a vast period of intense and widespread intrusive action. The alternation of sedimentation and volcanic activities in the Precambrian has rendered the task of determining the geological succession very difficult, but the generally accepted statement for the Quebec area of the Canadian Shield is as follows, in ascend ing order: Keewatin and Abitibi, Grenville (altered sedimentaries), Laurentian (granites and gneisses), Lower Huronian (Temiscam ing), Upper Huronian (Cobalt, Animikie), Keweenawan. That the area was transgressed by the sea after Precambrian times is shown by the considerable Palaeozoic outliers at Lake St. John (Trenton) and at James Bay (Devonian).
(2) The St. Lawrence Lowlands are bounded on the north by the southern edge of the Laurentian Plateau, approximately fol lowing a line joining the cities of Ottawa and Quebec. To the south-east the Lowlands abut against the great Champlain fault, stretching from Lake Champlain to Quebec City. This fault sep arates the nearly horizontal Palaeozoic strata of the Lowlands from the highly folded rocks of the Appalachian region, the most south erly of the province. The St. Lawrence Lowlands, therefore, form a triangle area of about I 0,cm sq.m., the three apexes of the triangle being Quebec City, Ottawa and the foot of Lake Cham plain. The area is a very level one, and is the chief seat of the population of the province and is the principal agricultural area.
The underlying strata of the St. Lawrence Lowlands are almost wholly of Ordovician age. They lie horizontally or at a slight dip. The ascending order of the Ordovician strata is : Chazy, Black River and Trenton limestones, Utica and Lorraine shales. The limestones are extensively used for building purposes and cement, and the shales for tile and brick making. Below the Ordovician in the most westerly counties is the Potsdam sand stone of Cambrian age. The plain is broken through by eight iso fated hills of igneous origin, along a line of fifty miles. They are known as the Monteregian Hills, taking the name from Mount Royal, the most easterly one. They range in height from 715 to 1,775 feet. Mount Royal at Montreal is 770 feet above sea level. They are the remnants or substructure of ancient volcanoes of probably Devonian age, and that each, though isolated, arose from the same subterranean source is indicated by their common alkaline characters, the main mineral types being nepheline syenite and essexite. The Lowland soil is very uniform, being chiefly the Leda clay deposited at the close of the Glacial Period in the sea which then covered eastern Canada and New England; the clay is so named from the abundance of Leda shells it contains, rep resenting species still living below the 3o-fathom depth in the Gulf of St. Lawrence. On the northern and elevated boundaries
of the Lowlands are found the Saxicava sands, so named from the abundance of the shells of Saxicava, representing species still living above the 30-fathom line in the Gulf.
(3) The Appalachian Region of the province of Quebec lies to the south-east of the line joining the foot of Lake Champlain and the City of Quebec, and thence south-east of the St. Lawrence river and its long estuary, and comprises the nine counties known as the Eastern Townships, the regions of Beauce, Temis couata and Matapedia, and the whole of the Gaspe peninsula. It is determined physiographically by the extension into Canada of the Appalachian mountain system of the eastern United States, and in the Eastern Townships consists of three ridges about 25 miles apart, the most southerly one, however, entering the prov ince for a short distance only. In this part of the province the mountain ridges have the general name of the Notre Dame moun tains. East of Levis, opposite Quebec City, the ridges sink to low levels for several hundred miles, but in the Gaspe peninsula they rise to considerable heights, several being over 4,000 feet. In the Gaspe peninsula they are known as the Shickshock moun tains. The wide valleys of the Eastern Townships in the western portion of the Appalachian Region comprise an important agri cultural, dairying and stock-raising area of the province. There are many lakes and streams in this area, and developed water powers have determined a number of manufacturing centres, the chief of which is the city of Sherbrooke.
Geologically, the western portion of the Appalachian Region consists of highly disturbed Cambrian and Ordovician strata, but not comparable with those of the same periods in the St. Law rence Lowlands. The mountain-building forces, however, have uplifted and exposed along the axes of the ridges the crystalline rocks of the Precambrian basement. The general volcanic and igneous action has also given rise to lines of weakness, permitting igneous intrusion. The most important intrusion is the "Serpen tine Belt," extending from near the Chaudiere river to the United States border in the county of Brome, a distance of loo miles, in which belt are found the valuable deposits of asbestos and chromic iron, the asbestos being an alteration product from the peridotite of the original igneous magma. In the Gaspe peninsula rocks of Silurian and Devonian age are largely devel oped, with smaller areas of Devono-Carboniferous rocks at the eastern extremity of the region, these latter rocks being the youngest known in the province.