GIOGNOSY.
Geognosy, already defined may be treated under the topics atmosphere, hydrosphere and litho Atmosphere may be considered as a gaseous envelope entirely surrounding the earth' and penetrating into the pores- of the outer crust. It consists essentially of .79 . per cent nitrogen and 21 per cent oxygen. Carbon dioxide (CO.) makes up about 0.08 percent, and water vapor, dust and 'rare gases are pres ent in. variable but small amounts. Nitrogen is very inert; but the oxygen, carbon dioxide and water vapor are all geologically active agents, the activities of which will be further discussed under dynamical geology, in the sec tion devoted to the work of the atmosphere. See ATMOSPHERE; WIND; METEOROLOGY.
The Hydrosphere, including the 'waters of the ocean lakes, rivers and cir culation, constitutes a second but only partial en velope surrounding' penetrating the earth's crust. Geologically such waters' are important agents both chenucally as solvents and mechani cally as a means of erosion, of transportation of sediments and of deposition. These activi ties will be more fully treatedin the section de voted to dynamical geology under 'the. topics of ground water, running water, oceans and lakes. See RIVERS ; OCEAN ; LAKE.
The Lithosphere is the solid crust of the earth. For the most part it consists of a few common chemical elements in great abundance and a great many rarer elements in extremely minute quantities. Below are given the eight leading elements' in the' order of their abund ance.
oxygen 47.07 Calcium 3 . 44 Silicon .. as .06 Pot.445Ufm....... c .. 2.45 Aluminum 7.90 Sodium Iron , .. ..... '4.43 Magnesium 2.40 Total It will be noted that the only useful metals in the list are iron and aluminum. Aside from these eight all the remaining known elements together make up less than 2 per cent of the crust. Estimates express zinc, tin and lead in the third or fourth decimal place, copper in the fourth or fifth, silver in the sixth or seventh, and gold in the seventh or eighth. (See CHEMISTRY). These elements are combined in various ways to make up a large number of minerals, the most abundant of which are the feldspars, quartz, the pyroxenes, the amphiboles, the micas, calcite and kaolinite. Such minerals are definite chemical compounds. See FELDSPARS MINERALOGY; QUARTZ ; etc.
These minerals aggregated mechancally in varying proportions constitute. rocks, which from minerals in not having a definite chemical composition. Three great classes of rocks are recognized. Certain ones of these, Icnown as igneous, have cooled and solidified from a hot fluid mass called a magma. The igneous rocks are further subdivided on a two fold basis, that of mineralogical content and that of texture. Nd attempt can be made here
to outline the classification. Suffice it to men tion granite, syenite, diorite, gabbro, rhyolite, diabase and trap as among well-known igneous rocks: (See ROCKS; IGNEOUS ROCKS; PETROL OGY ; GRANITE; GABBRO ; etc.). Another great group of rocks are those laid down as deposits or sediments by water, wind,. or glacial ice.. They are usually stratified and are called sedi mentary rocks. . These may be further classified as aqueous, eolian and glacial; or as mechanical and organic. The common types are sandstone, shale, limestone and conglomerate. Sedimentary rocks are usually said to be de rivative, inasmuch as they have all been de veloped by the weathering of igneous rocks by processes to . be outlined under dynamical geology. (See ROCKS ; SEDIMENTARY Roca; SANDSTONE ; SHALE ; etc.). Both igneous and sedimentary rocks, by undergoing great heat and pressure or shearing stresses, may be altered (metamorphosed) to other rocks. Such altered rocks either of igneous or sedimentary origin are called metamorphic rocks, the most com mon types of which are marble, slate,_quartztte, schist and. gneiss. (See. ROCKS; METAMOR PHISM ; MARBLE; PETROLOGY; etc.). Clays, sands and gravels, though unconsolidated are con sidered rock by the geologist and so classified. To this unconsolidated surficial material usu- . ally denominated soil is often given the name mantel-rock or regolith, as distinguished from solid rock or bedrock. See Son.
Interior of the The above descrip: tion applies to what is usually known as the crust of the earth, that is, the outer shell of a few miles regarding which information is available. Concerning the interior of the earth,. sometimes called the pyrosphere or the centre. sphere, little is really known. It was long thought that the interior was molten, the belief being based on the extrusion of lavas, the ex istence of hot springs and the known rate of increase of temperature in deep mines. But it is a well-known principle that for most substances increased pressure increases the melting point, and it is quite possible that the enormous pres sures prevent the rocks from becoming molten. The rigidity of the earth has convinced physi cists that, whatever its temperature, the in terior acts as a solid. mass. Of the composi tion of the interior next to nothing is known. The fact that the earth as a whole has a higher specific gravity than its surface rocks has led many to suppose that the interior consists of very heavy minerals or metals, perhaps iron, or. . even precious elements. This, however, is mere speculation.