The rocks are most conveniently classified according to their mode of origin, into three main groups, igneous, sedimentary, and metamorphic. The igneous rocks are those which have solidified from a molten magma, like lavas, granites, etc. The sedimentary rocks are those which represent accumulations of fragments or grains, derived from various sources, usually the weathering of other rocks, and deposited by such agents as water, wind and organisms. Metamorphic rocks are those which were originally either igneous or sedimentary, but have been altered by the actions of heat, pressure and water, so that the primary character has been changed, often to such an extent as to be obscured. Rocks once formed in any of the above ways are being constantly altered in character by the various processes of nature. Those exposed on the surface are weathered to pieces, and the fragments are transported by wind or water to accumulate elsewhere as sedimentary rocks. Those buried deep beneath the surface are affected by the high temperature and pressure of the depths of the earth and thus metamorphosed. For instance a granite exposed on the surface is slowly weathered, some parts being carried away in solution by the rain water, others less soluble remaining as grains of quartz, mica or kaolin. These are transported by water and sorted, the finer kaolin being carried to still and deep water, the quartz and mica accumulating in some lowland as sand. This sand will in time be cemented to a sandstone, later slowly buried beneath the surface. If buried deep it will feel the effect of the interior temperature, which increases as one goes down at the rate of one degree F. for every 50 feet. If this should be in a region where folding and mountain-making takes place, the material under the folds would be melted (because of the relief from pressure which would permit the high temperature to act freely) and become igneous rock, either coming to the surface as lava, or remaining below the surface and making a granite or similar rock; while the sedimentary material not melted but near enough to the molten material to be affected, would be metamorphosed, in this case to a quartzite. Much of the interest and profit in studying rocks, will come from the understanding which they will give as to the history of that particular part of the earth’s crust where they are found. Igneous Rocks