_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.