_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.