LKSSONS IN UKOUXiY. 



LESSONS IN GEOLOGY. IX 



LITHOLOGY-PETBOLOGY GEOLOGICAL TERMS. 



UNDER the general term Geognosy is included all that can be 

 said in relation to the structure of the matter composing the 

 earth's crust. Such a description readily permits of a sub- 

 division. 



The rock-masses may be treated of as to their mineral, 

 their internal structure, and those characters which may be 

 determined by handling specimens, such as texture, construction, 

 hardness, etc. All this is Lithoiogy. 



Petrology describes the larger charac- 

 teristics of rook ; the relative positions they 

 occupy, and the disturbances they have 



larger quantity war* treated in the same manner, the b**alt 

 returned to its former stony appearance; bat in doing to, it 

 WM observed that the molten mass congregated into little boll*. 



Neither of these sub-divisions treats of 

 rocks urranirt'ii in geological groups. Hence 

 this forms a third part of the subject, and 

 to enable us to classify rooks chronologi- 

 cally that is, to determine which rocks were 

 deposited at or about the same time we 

 mast appeal to their organic contents. The 

 mineral composition is of little service. 

 For instance, two sandstones, identical in 

 their composition, may be found, which we 

 know must have been formed at two periods 

 separated by vast intervening ages; and 

 this wo discover by their fossil contents. Hence a study of 

 fossils is of the greatest importance, and this has received the 

 name of Palaeontology. 



Mineralogy is a further sub-division of Lithoiogy. 



Lithoiogy, as wo have said, treats of the mineral composition 

 of the rooks, but does not enter into the chemistry, etc., of these 

 mineral elements. Thus we find the remark we made in the 

 opening lesson is true. To be a perfect geologist we must 

 begin with Chemistry. This t3aches us the elements whereof all 

 bodies are composed, and the laws of their combination. A 

 step higher, and we reach Mineralogy, by which we are taught 

 how those elements are arranged in bodies contained in the 

 solid crust of the earth ; the forms in which minerals appear, 

 and their characteristics. Lithoiogy groups these minerals into 

 rocks, and forms the third division of our 

 subject ; Petrology tells how the rocks lie on 

 the surface, their contortions, interstratifi- 

 cations, etc. ; while Palaeontology, from the 

 fossils the rocks contain, groups them ac- 

 cording to their ages, and writes a history 

 of the animal life of the globe. Minera- 

 logy will be treated separately; and for a 

 further description of the minerals we shall 

 have occasion to name, we refer the reader 

 to the forthcoming lessons. 



Technical Terms descriptive of the Struc- 

 ture and Internal Characters of Rocks. 

 Although the particles of a mineral may be 

 arranged in a symmetrical order, forming a 

 crystal, this formation never extends to large 

 masses. No crystal has been discovered 

 more than one or two feet long ; but when 

 a rock is composed of particles which are either crystals or frag- 

 ments of crystals, it is said to be crystalline. Such is the struc- 

 ture of loaf-sugar. The granites and many of the primary 

 rocks exhibit it. Occasionally a uniform structure is developed 

 in large masses of rock ; this is always columnar (Fig. 14), as in 

 the basaltic rocks of the Giant's Causeway, in the north of Ire- 

 land, and in Fingal's Cave, on the opposite shore of the Isle of 

 Staffa. 



The number of the sides of the columns varies from three to 

 ten. They fit so closely to each other that the blade of a knife , 

 is with difficulty inserted between them. When broken hori- 

 zontally they exhibit a mnchoidal fracture ; that is, the surface 

 of one piece is concave, the corresponding surface of the other 

 being of course convex like a ball-and-socket joint. 



The origin of the columnar structure of rocks was proved ex- 

 perimentally by Mr. Gregory Watt. He caused a quantity of 

 basaltic rock to be fused in a blast furnace. When a small 

 portion of this was removed and allowed to cool, it assumed a 

 vitreous character ; that is, like glass, but not transparent. If a 



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it. 



and these increased in size by the continuous addition of 

 matter to their external coating. In doe time these 

 touched each other; bot M their increasing capacity still 

 tinued, they were squeezed together ; and M the expansive force 

 progressed, their curved surfaces were flattened. Presuming the 

 force in each ball to be exactly the MUM, by this promo 

 (six-sided) short prisms would be produced. Thu will 

 be sufficiently evident from Fig. 1 .'.. 



For some reason or other these spheral 

 arranged themselves one above the other, so 

 that a straight line would paes through their 

 centres. This explains the formation of the 

 columnar structure. 



Certain green stones are found which ap- 

 pear in spherical masses. When exposed to 

 the weather they are liable to exfoliate-, 

 that is, one coat after the other peels off, 

 thus demonstrating their concretionary cha- 

 racter, or that they were made by successive 

 layen accumulating round a nucleus, pro- 

 bably in the same manner as the globules 

 of basalt, in which the columnar structure 

 originated. 



All masses of rock are intersected by 



regular sets of cracks, which are termed jointt. Without these 

 it would be difficult to quarry stratified rocks, and impossible to 

 hew out hard primary rocks. 



Joints generally traverse rocks at right angles to each other. 

 In granite quarries three seta frequently cut the stone into cubical 

 masses. 



Their existence can only bo accounted for by supposing they 

 result from the shrinkage of the mass upon its consolidation. 

 When those joints are very close to each other, and pass 

 through the rock- mass in the same direction, the phenomenon 

 is termed cleavage, and the rock splits into slates. Some 

 stratified rooks readily divide into horizontal slabs, the pianos 

 of division coinciding with those of stratification. Such slab* 

 are called flags or Jlagstones. 



When the rock exhibits a disposition to 

 break up into thin leaves, it is said to be 

 U> minated. Shales are clayey deposits which 

 were once soft and plastic, but upon drying 

 became intersected by numerous joints, 

 which cause the mass to split up into small 

 pieces. 



Crystalline rocks are frequently found 

 possessing a laminated structure ; they are 

 then schists, or of a schistose structure. 

 Occasionally the term foliated is applied to 

 rocks of this nature, from the leaf -like man- 

 ner in which the layers overlieeach other. 

 Fi.^i'c is a general term which characterises 

 all rocks which show any tendency to break 

 up into small parts. 



When a layer of rock reposes on a stratum 

 of different material, its under surface must 

 partake of smoothness or inequality of that stratum ; this is de- 

 nominated bedding. A stratum of rock is sometimes called a bed, 

 but the word is becoming geologically obsolete. Seam, unfor- 

 tunately, has two meanings ; sometimes it is used to indicate the 

 lino which marks in a section the junction of two strata, and 

 also it is applied to thin layers of mineral matter which traverse 

 a series of strata. For example, thin beds of coal, a few inches 

 or even feet thick, ore frequently found at intervals in many 

 strata ; these would be called seams of coal. It is evident how 

 the word has become applicable to each ease. The thin bed was 

 considered the line which divided the two layers of strata. 



The above terms are descriptive of the structure of rock- 

 masses. On examining a specimen in the hand, a closer inspec- 

 tion reveals to us its internal characters. If it be made up of 

 the same kind of matter, the rook is homogeneous ; when its 

 particles are in grains, perceptible to the touch or plainly visible, 

 the specimen is yranular. Occasionally the material is arranged 

 in fibres, as asbestos ; it is then Jibroiu. When the fibres are 

 distinct, long, fine crystals, it is said to be acicuiar (needle-like) 



15. 



