THE EARTHS CRUST. 5 



The study of the materials and general structure of the Earth's 

 crust is sometimes termed Geognosy, while the particular study of 

 the rocks is termed Petrology or Lithology. Into the composition 

 of the Earth's crust about sixty-four elementary substances are 

 known to enter, the most important of which are Oxygen, Hydrogen, 

 Chlorine, Carbon, Sulphur, Silicon, Calcium, Magnesium, Sodium, 

 Potassium, Aluminium and Iron. While changes of various kinds 

 are continually in operation, no new materials are added, with the 

 exception of Meteoric stones and dust. Meteoric dust may include 

 volcanic material, such as that of Krakatoa, as well as Cosmic dust. 

 Meteorites are of interest to the geologist as affording possible 

 evidence that the composition of the Earth's interior is to some 

 extent metallic. It has even been suggested that they were ejected 

 from the earth during its early volcanic phase. Twenty-four of the 

 commoner elements in the Earth's crust have been recognized in 

 them. Those called Siderites yield from 80 to 95 per cent, of iron ; 

 while the Siderolites and Aerolites are of a more stony character.' 



The Rocks forming the earth's crust are arranged into four 

 classes, according to their method of formation : — 



1. Aqueous or Sedimentary Rocks (in old times known as 

 Neptunian). — These include most rocks deposited under water, 

 as Gravel, Sand, Clay, Shale, Marl, Limestone, etc. 



2. SuBAERiAL or TERRESTRIAL RocKS. — Thcsc includc dcposits 

 formed or accumulated on land areas, as Peat, Coal, and Blown Sand. 



The rocks belonging to these two classes are usually stratified 

 or arranged in layers ; and they are fossiliferous. 



3. Igneous or Eruptive Rocks (formerly termed Plutonian). 

 — These include the rocks which have been intruded in a molten 

 condition as bosses and dykes amidst the stratified rocks ; and 

 those which have been poured out as lava-flows or ejected as ashes 

 from volcanic centres. They are, as a rule, crystalline, unstrati- 

 fied, and unfossiliferous, and include Basalt, Diorite, Trachyte, 

 Felstone, Porphyry, and Granite. 



4. Metamorphic Rocks. — These include rocks which by con- 

 tact with igneous rocks, and by excessive pressure and disturbance, 

 have undergone great alteration, so that their original character is 

 much changed and their stratification is frequently obscured. Their 

 structure is often schistose or foliated — the mineral components 

 being arranged in separate layers. Quartzite, Mica-schist, Gneiss, 

 some kinds of Marble, and Slate belong to this class. 



David Forbes proposed the name Ingenite (signifying created 

 within or below) for the granitic, metamorphic, and igneous rocks ; 

 and Derivate for all sedimentary, subaqueous, and subaerial 

 accumulations, since directly or indirectly the latter are all derived 

 from the destruction of the former. 



There can be no doubt that the Aqueous rocks, as a rule, 

 were formed and deposited on the ocean-bed in the same 



1 L. Fletcher, P. Geol. Assoc, vii. 351. 



