Economic Importance of the Igneous Terranes.—The igneous rocks, as previously noted, are such as have cooled from fusion. On the cooling of magmas various minerals are formed, most commonly silicates, and except in a minor way in connection with the weaker stages of volcanic activity and the slow cooling of the rocks, there does not seem to be any marked tendency towards the concentration or segregation of metallic minerals or ores. Although igneous rocks do contain gold, silver, copper, etc., and a large variety of the rarer metals, they are widely disseminated. As is well known, however, igneous rocks are in some instances of value for the metallic mineral, gems, and ores associated with them, but in the great majority of instances at least, and as a rule, these minerals and ores are the result of subsequent changes and owe their origin mainly to deposition from heated, percolating water. Rich ore bodies frequently occur on the borders of igneous dikes, and in fissures and cavities in igneous rocks, but the process by which they have been formed is similar to that leading to the concentration of mineral matter in metamorphic rocks, and will be referred to later.
The igneous rocks themselves furnish desirable building stones, such as granite, diorite, porphyry, diabase, etc. With the exception of granite and the nearly related diorite, these have not as yet been extensively utilized in North America. Certain of the igneous rocks have been altered to serpentine, which on account of its pleasing green colour and the ease with which it can be cut and
polished furnishes a stone valuable for interior uses. It is also employed, usually with a rough surface, in the construction of exterior walls of dwellings, gateways, etc. Large bodies of serpentine occur at a number of localities in the Atlantic mountains from Pennsylvania and Maryland northward, including eastern Canada, and also over extensive areas in the Pacific mountains, particularly in California, Washington, and Alaska.
The principal ores and minerals of commercial importance in the igneous rocks are native copper, as in northern Michigan; copper pyrites, as at Butte, Montana; gold, at many localities, including the Treadwell mine, Alaska; opal, which is mined on a small scale in Idaho and Washington. In practically all these instances, and numerous others that might be enumerated, the substances referred to have been deposited from solution in cavities in the rocks or have replaced other substances, and are due to what is termed above chemical concentration.
Economic Importance of the Sedimentary Terranes.—The sedimentary rocks are composed principally of fragmental material derived from the disintegration of older rocks transported and deposited mechanically, and resulting in the formation of sandstone, shale, etc., and of organically concentrated material, such as shells and corals, which form limestones. The deposits originating in these ways furnish excellent building stones, the principal classes being sandstones and limestones. These occur widely throughout North America, and in formations of all ages subsequent to the Archean. The sandstones were deposited near the shores of the seas, or in lakes, and the limestones principally in moderately deep oceans.
Sandstones occur largely in the Cambrian formation on the south shore of Lake Superior and about the borders of the Adirondack hills of New York. They are usually red or reddish-brown rocks, and their pleasing colours, durability, even grain, and the readiness with which they may be broken in any direction make them desirable building stones.
The Newark system, extending in detached areas from
Nova Scotia to South Carolina, contains immense quantities of brown and gray sandstone, which have been extensively quarried, particularly in the Connecticut Valley, New Jersey, Pennsylvania, and Maryland, and largely used in Atlantic coast cities. The Carboniferous and Devonian sandstones, usually of a gray colour, of Pennsylvania, Ohio, and neighbouring States, are largely used in the cities of the interior portions of the United States. Extensive deposits of Mesozoic and Cenozoic sandstones occur throughout the Pacific mountains, and afford a practically unlimited supply of good building material, which as yet has been but little utilized. The colours of sandstones vary from bright red through brown-yellow to gray, and in some cases are nearly white, depending largely on the condition of the iron present. The red rocks are dyed with ferric oxide; the brownstones contain iron, frequently in the cementing material that unites the grains, in various stages of oxidation and hydration; the gray stones may also contain iron, but if present it is in union with organic matter, as the ferric carbonate, for example. The Cambrian and Newark sandstones are prevailingly of some shade of red, for the reason that not enough organic matter is present to change the iron to a carbonate.
The sandstones when of an even fine grain and not too hard, are suitable for sharpening tools, and large quantities of grindstones, whetstones, etc., are made from them, as on the Lake Huron shore of Michigan, in Ohio, etc. Other sandstones, practically free from iron, are used in the manufacture of glass. The best example of "glass sand" is the Sylvania sandstone of southeastern Michigan. Unconsolidated sand is largely used in mixing mortars and cements, for smoothing stones used for architectural and monumental purposes, as foundry sand in making moulds for casting, and many other ways. Seaward from where sand is being deposited we find in the present oceans that as a rule fine bluish or greenish mud occurs, and still farther seaward, except where coral-polyps thrive, usually at a distance of 100 miles or more from land, the bottom is composed of calcareous mud or ooze. The
sand and mud are derived from the land, and if consolidated form sandstone and shale. The calcareous ooze is derived from the life of the sea, largely minute lime-secreting foraminifera, together with shells of molluscs, and in the vicinity of coral islands or reefs the hard parts of coral growth are added. That is, the calcareous oozes are formed by the concentration of calcium carbonate through the vital action of animals and to a less extent of plants. Such material, if consolidated, would form ordinary limestone.