CHAMBERS'S INFORMATION FOR THE PEOPLE. 



has much the same composition as quartz, and is 

 very plentiful in chalk. The granular rock brought 

 from Aberdeen and elsewhere, so beautiful when 

 polished, is called granite, from its being com- 

 posed of grains of other rocks. It has two chief 

 varieties, the gray and the red, according to the 

 prevailing mineral in its composition. It is com- 

 posed of quartz, mica, and felspar. It composes 

 the mass of some of the chief mountain ranges, 

 and forms part of some of the grandest scenes in 

 nature. It is in general of igneous origin, but is 

 certainly not always such ; its origin is the subject 

 at present of much controversy. The particles 

 that glitter like silver in the granite are pieces of 

 micaf which is so named because it shines. A 

 mineral very like mica in appearance, but different 

 in composition, is called talc, from its feeling 

 somewhat greasy or tallowy when touched. 



The molten matter that flows from volcanoes is 

 called lava ; % pumice-stone* is the cinder of such 

 discharges ; while the ashes that are thrown into 

 the air are called scoria. In geologic times also 

 there existed volcanoes from which lava issued ; 

 the rock this lava formed is called trap? from 

 lying in stair-\\\x. masses, as it flowed from the 

 mountain ; and one kind, whinstone, which is 

 much used for roads. A common variety is known 

 as greenstone, from its colour, of which Salisbury 

 Crags, near Edinburgh, are composed. Another 

 variety is called basalt, and is generally found in 

 columns standing close together, which often form 

 wonderful natural scenes, such as Fingal's Cave 

 and the Giants' Causeway. Another variety is 

 porphyry, 6 so called from its frequent purple colour, 

 and is easily distinguished by its granular appear- 

 ance. A kind of light porous rock, formed of 

 cohering volcanic ashes, is known as trap-tuff, or 

 tufa, a word that comes from Italy, the seat of so 

 much volcanic action. 



STRUCTURE OF ROCKS. On examining the 

 rocks forming the crust of the earth, we find that 

 they may be divided into two great classes the 

 stratified, or those deposited in strata or layers ; 

 and the unstratified, or those not so formed. Sand- 

 stone and slate are stratified rocks ; granite and 

 trap are unstratified. 



I. Stratified Rocks. Any thin deposit of rock 

 is called a layer, from its having been laid down 

 under water; a band, from its being like a thin 



band ; a bed or a stratum, when of greater thick- 

 ness, from Latin sterna, to spread ; and a seam, 

 when of a peculiar character as compared with 

 the rocks near it, as a seam of coal Stratum, 

 with the plural strata, is the general term for any 

 layer of rock, and hence all rocks in layers are 

 said to be stratified. Rocks that split up into 

 very thin layers, a great number being included 

 in the thickness of an inch, are said to be lami- 

 nated, and the thin layers are called lamina ; 

 these are formed by deposition. Slate-rocks have 

 a remarkable tendency to split or cleave in one 

 direction, which is called their cleavage. This in 

 general does not coincide with the lamination, and 

 may be at any angle. It is by the cleavage that 

 the slates of commerce are separated. Some 

 attribute this phenomenon to pressure, others to 

 heat. Gneiss and other rocks have a tendency to 

 split or foliate 1 into thin layers of different miner- 

 alogical character, as quartz, mica, and such like. 

 This is called their foliation, and must be distin- 

 guished from their stratification and also from cleav- 

 age, which exists in rock of one mineral composi- 

 tion. Granite and other rocks are often split into 

 large masses, more or less cubical, along certain 

 lines at equal or varying distances. These lines of 

 separation are called joints. They occur much in 

 igneous rocks, as in basalt. These words should 

 be carefully distinguished : thus shales are lami- 

 nated ; schists, foliated ; igneous rocks, jointed ; 

 slates, subject to cleavage. When a rock is com- 

 posed of rounded pebbles or boulders imbedded 

 in other matter, it is called a conglomerate, and 

 sometimes, from its appearance, pudding-stone or 

 plum-pudding stone. 



2. Unstratified Rocks. Unstratified rocks 

 assume various forms, according as they have 

 been shot up amongst the stratified rocks ; for, as 

 we shall afterwards see, they have been erupted 

 from volcanoes. Very often, like most volcanic 

 substances, they axe porous or cellular, like pumice- 

 stone ; frequently they form gigantic columns, when 

 they are said to be columnar, like basalt ; and 

 often they are found in large globular or spherical 

 masses, like bombs or cannon-balls. 



DISPOSITION OF ROCKS. i. Stratified Rocks. 

 When rocks lie parallel to the horizon, they are 

 termed flat or horizontal, as A, in the following 

 section ; when at an angle to it, they are said to 



u 



Y W v~ 



Section of the Different Kinds of Strata. 



A, Horizontal and unconform- 



able rocks. 



B, Inclined strata. 



C, A slip. 



D, A disrupting mass. 



E, Overlying trap. 



F, Interstratified trap. 



G, A dike. 



H, Bent and rolling strata. 



I, A basin or trough. 



K, A ridge of rocks. 



L, The dip of the rocks, 45*. 



M, An outcrop. 



N, Veins. 



O, A fault. 



P, Contorted or twisted strata. 



Q, A seam. 



R, Rocks tilted up. 



S, Columnar basalt 



T, Conglomerate. 



U, Limestones. 



V, Granite, disrupting. 



W, Trap. 



X, Sandstones. 



Y, Coal. 



Z, Silt or gravel. 



be inclined or dipping, as B ; when one end has 

 been thrown up by some other mass, they are 



1 From Latin mice, to* shine. * From Swedish trappa, a stair. 



* From Latin lava, to lave or flow. 6 From Greek porphyra, purple. 



* From Latin pitmex, -ids, cinder. 



18 



said to be tilted up, as R ; when so much inclined 

 as to be straight up and down, they are said to be 

 perpendicular, or to stand on edge. When inclined 



1 From Latin folium, a leaf. 



