IX. The occurrence of minerals

The minerals occur most commonly in a particular group of crystalline rocks, formed far beneath the surface of the earth, at high temperatures, and usually from liquid rock. Everyone is familiar with the phenomena of vulcanism. Volcano is a mountain having openings or vents in the earth's crust through which lava, cinders, water and gases pour out. The lava comes from beneath the surface, but much molten rock never sees the light of day. It is such lava, buried beneath the surface, protected from rapid cooling by a layer of other rocks above, that cools and crystallizes into rocks which contain the primary minerals.

Geologists have accumulated abundant (much) evidence that under surface conditions rocks are relatively easily altered. The process of alteration is called rock-weathering.

X. Sedimentary rocks

It is on the sea floor that new rock formations are produced from the assorted particles derived from the land. In addition, the dissolved solids which have been carried from the land into the sea may be extracted from solution by sea organisms that may build their shells and other body structures of calcium carbonate or silica. When these organisms die, their shells and hard structures are laid on the sea floor, thus forming layers of carbonates, although some portions of these layers are formed by the physical condition whereby dissolved carbonates are deposited directly from sea water with an increase of temperature. Some solids remain in solution, thus increasing the salinity of the ocean waters.

Sedimentary rocks are divisible into clastic and non-clastic types. Clastic rocks, such as conglomerate, sandstone, arkose and shale are made up mainly of the broken fragments of pre-existing rocks which denote mechanical disintegration of source materials. The composition and degree of rounding of the clastic grains show the extent of sorting and reflect the effects of transportation. Applying knowledge of modern conditions of weathering, transportation, and sedimentation, study of the composition and texture of ancient sedimentary rocks permits deductions as to conditions under which they were formed.

Non-clastic sedimentary rocks comprise materials deposited by chemical precipitation or through the agency of organisms. Salt, gypsum, anhydrite, and some deposits of limestone and dolomite have been deposited by direct chemical precipitation from marine or inland waters made supersaturated by evaporation. Limestone is quantitatively much less important than shale and sandstone among sedimentary rocks of the world, but it is the chief non-clastic type. Kinds of limestone, classed on the basis of composition and texture, are almost without number, and each represents somewhat different conditions of making.

Organic remains. A characteristic feature of sedimentary rocks that is vitally important to historical geology is the common occurrence of fossilized remains of organisms. Fossils are the remains or traces of animals or plants preserved in rocks. Before the time of modern science, men regarded shells, bones, or leaf imprints discovered in rocks as freaks of nature. Shortly before 1800, the discovery was made that fossils of a given bed or group of beds are characteristic of it and serve to distinguish it from other fossil-bearing rock layers.