GEOLOGY 



has been generally accepted as 

 about 1 F. for every 53 ft. of des- 

 cent from the surface ; according to 

 some measurements the increase is 

 only 1 F. for every 80 ft. of de- 

 scent. Even if the rate becomes 

 slower with increasing depth only a 

 few miles below the surface the 

 heat must be intense. In addition 

 to its other names this central mass 

 has, therefore, been called the 

 thermosphere, by those wishing to 

 direct attention to the important 

 consequences of its high tem- 

 perature. 



Internal Composition 



The second fact proved about 

 the central core is that its materials 

 are much heavier, bulk for bulk, 

 than the rocks of the earth's crust. 

 These rocks weigh from about 2-| to 

 3 times as much as an equal bulk of 

 water ; the whole earth, however, 

 weighs nearly 5| times that of an 

 equal bulk of water. The material 

 in the interior of the earth is there- 

 fore more than twice as heavy as 

 the rocks of the crust. This fact is 

 probably due to the high proportion 

 in the interior of various metals, 

 especially iron and nickel. The 

 central mass is different in composi- 

 tion from the rocks of the crust ; its 

 material has been termed Gei'te, i.e. 

 earth-rock. It is probably similar 

 to the largest meteorites, which 

 consist of iron with from 6 p.c. to 

 10 p.c. of nickel ; hence probably 

 both the bulk of the meteoritic ma- 

 terial and of the earth consist of 

 iron and nickel. That the litho- 

 sphere is only about 50 m. thick, 

 below which the earth consists of 

 geite, is probable from the phe- 

 nomena of earthquakes and the 

 distribution of radium. 



THE ROCKS OF THE EARTH'S 

 CRUST. The lithosphere is the part 

 of the earth with which the geolo- 

 gist is most concerned. It consists 

 of rocks, which are masses of fairly 

 uniform coherent material, such as 

 granite and sandstone. Rocks sup- 

 ply the materials for the history of 

 the earth, as each of them retains 

 characters which indicate the con- 

 ditions under which it was formed, 

 and often reveal its age. Study of a 

 rock will usually determine whether 

 it was formed on land or sea ; if on 

 land, whether under a moist or a 

 desert climate ; if in the sea, 

 whether near the shore or in a deep 

 ocean ; and if formed beneath the 

 earth's surface, at what approxi- 

 mate depth. 



Rocks are of two main kinds. 

 Those of the first kind are formed 

 by the solidification of molten ma- 

 terial ; they are sometimes called 

 primary, as they haye been formed 

 directly from the molten constitu- 

 ents of the earth ; and as their 

 molten condition was due to in- 



3474 



tense heat they are also called ig- 

 neous. In some cases these rocks 

 have solidified on the surface of the 

 earth in sheets which have been 

 discharged from volcanoes. They 

 are then called volcanic rocks. If 

 they solidify very quickly they 

 form glass such as obsidian ; if 

 they cool very slowly and under 

 heavy pressure, the whole of the 

 material will solidify in a crystal- 

 line state. Under intermediate con- 

 ditions a primary rock may be 

 composed of a mixture of crystal- 

 line constituents and glass. Rocks 

 which have consolidated at a con- 

 siderable depth are known as Plu- 

 tonic rocks (after Pluto, the god of 

 the infernal regions) and owing to 

 their slow cooling under great pres- 

 sure none of their material can 

 solidify as glass ; they consist 

 wholly of crystalline constituents 

 and are accordingly described as 

 holocrystalline. 



Simple and Compound Minerals 



The plutonic rocks consist, 

 therefore, of an aggregate of crys- 

 talline materials, each of which is a 

 simple mineral. The term mineral 

 is used in a broad sense, as in 

 mineral kingdom, to include all the 

 inorganic constituents of the earth. 

 The simple minerals or mineral 

 species are those which have a de- 

 finite chemical composition, which 

 often have a regular shape and can- 

 not be broken up by any simple 

 mechanical processes into other 

 minerals. Compound minerals, 

 such as coal, iron ore, slate, granite, 

 etc., are, on the other hand, mix- 

 tures of simple minerals ; granite, 

 for example, may be seen by the 

 naked eye to consist of a mixture of 

 simple minerals which can be sepa- 

 rated by hand when the rock is 

 crushed. 



Rocks are usually composed of 

 mixtures of simple minerals, by 

 the identification of which their 

 composition and history can be de- 

 termined. The lithosphere must 

 originally have consisted solely of 

 primary rocks ; the surface layer 

 was decomposed by air and water 

 and the fragments used as the con- 

 stituents of a new generation of 

 rocks. Because in them the ma- 

 terial is used for a second time they 

 are called secondary rocks. As they 

 consist of broken fragments they 

 are clastic ; the fragments are 

 large, such as pebbles in a con- 

 glomerate, of coarse grains in a 

 sandstone, and of particles so 

 minute that they cannot be seen by 

 the naked eye in a clay or shale. 

 Sedimentary Rocks 



As these fragments are deposited 

 as sediment, the rocks they form 

 are called sedimentary ; as these 

 rocks are deposited in layers, each 

 of which is a stratum, the second- 



GEOLOGY 



ary rocks are stratified. Most of 

 them having been laid down by 

 water, as on the sea floor or on the 

 bed or banks of rivers, they are 

 therefore called aqueous rocks. 

 During their deposition remains of 

 animals and plants are embedded 

 in them and preserved as fossils. 

 Secondary rocks, then, are clastic, 

 sedimentary, stratified, and often 

 fossiliferous. Primary rocks, on 

 the contrary, consist of original 

 glassy or crystalline constituents ; 

 they are therefore not clastic, and 

 they are unstratified, igneous, and 

 ufifossiliferous. 



In addition to the stratified 

 rocks made of sediments there are 

 some composed of the shells, 

 skeletons or hard tissues of various 

 animals and plants. The most im- 

 portant representative of this 

 group is limestone ; it consists of 

 carbonate of lime which has been 

 extracted from water by corals, 

 shell fish (mollusca), calcareous 

 plants, etc. Some organisms secrete 

 shells and skeletons of silica, and 

 their remains form beds of chert or 

 flint. Some plants extract iron 

 from water and they deposit 

 layers of iron ore. Some animals 

 extract phosphoric acid and form 

 shells and bones of phosphate of 

 lime ; they give rise to phosphatic 

 limestones, which are of great value 

 as a source of manures. Another 

 group of stratified rocks is de- 

 posited chemically, generally as 

 residues left by the evaporation of 

 water; such are beds of rock salt 

 and of various potash salts. 

 Metamorphic Rocks 



Intermediate in character be- 

 tween the primary and secondary 

 rocks is a third group which con- 

 sists of rocks that have been al- 

 tered by heat, or superheated 

 steam, or the injection of veins of 

 molten rock, or by intense pressure. 

 These rocks often retain their ar- 

 rangement in strata, but their con- 

 stituents have been crystallised 

 and any fossils that may have oc- 

 curred in them have been de- 

 stroyed. These rocks have been so 

 thoroughly altered that they are 

 known as metamorphic. They 

 have been produced from both 

 secondary and primary rocks.They 

 have generally been formed at 

 great depths below the surface, 

 and have been exposed by uplift in 

 mountain chains or by the removal 

 of the rocks which once covered 

 them. The peninsular part of India, 

 most of Scandinavia and Finland, 

 and the western part of Australia 

 each consists essentially of a large 

 exposed block of these once deep- 

 seated metamorphic rocks, as- 

 sociated with igneous rocks which 

 have been forced into them from a 

 still deeper zone. 



