LESSONS IN GEOLOGY. 



177 



Fig. 25 



LESSONS IN GEOLOGY. XII. 



IGNEOUS BOCKS 



As may havo boon gathered from a previous chapter, igneous 

 .in Mib.livi.li' I by Borne geologists into Volcanic and 

 1'lutu!.: : i i uit is, into igneous rooks of a recent dato of forma- 

 tiun 1:1 fact, those whose existence is due to volcanic action 

 ami t luis.- whi'-h have been formed at groat depths in the earth 

 at a muh more distant epoch. This division, however, is very 

 unsatisfactory, for there are many rocks which it is impossible 

 in cither of those divisions, and therefore the best classi- 

 fication which is now generally accepted is : 



1. (ii-tinilic AVfcj. 



J. Trappean Rocks. 



;i. Volcanic Rocks. 



The study of these rocks is attended with much difficulty. Not 

 only is their composition somewhat complex, but two rocks 

 having identically the same composition often 

 present a totally different appearance, owing to 

 the fact that they were cooled under different cir- 

 cumstances. We have proof of this in the case 

 of molten glass : slight differences in the man- 

 ner of cooling materially affect the character of 

 the glass, so that from the same molten mass 

 a transparent glass, opaque slag, or a vitreous 

 stone may be obtained. Moreover, they have no 

 definite order of superposition, but are found 

 traversing the stratified strata 

 in veins, which often intersect 

 masses of igneous rock already 

 solidified. They contain no or- 

 ganic remains by which their 

 relative ages can bo satisfac- 

 torily established. Hence they 

 do not present that interest to 

 the geologist with which the 

 fossiliferous strata are invested. 



It is but right to warn the 

 student that the igneous origin 

 of these rocks is questioned. 

 The main reasons for supposing 

 that the operation of water has 

 been material to the production 

 of the igneous rocks are (1) 

 because we find water now pre- 

 sent in them (minerals of the 

 aluminous class almost invari- 

 ably contain it) ; and (2) it is 

 urged that sea and rain water, 

 in percolating though the crust 

 of the earth, must at length 

 reach the regions of molten 

 matter. However, in spite of 

 these reasons, which are not in- 

 superable objections, the igne- 

 ous origin of these rocks is 



generally received. This the reader will gather for himself, as 

 we shall find granite and other primary rocks in positions which 

 they could only have occupied when in a molten state. 



THE VOLCANIC ROCKS. 



These, as their name indicates, are supposed to have their 

 origin in volcanic action. They are being produced to-day by 

 the various burning mountains in action, which eject from their 

 craters scoriae, ashes, lava, and other varieties of volcanic rocks. 

 These rocks are found associated with the uppermost three 

 formations the post-tertiary, the tertiary, and the chalk but 

 below this their presence has not yet been discovered. 



It is sufficient, for a brief notice, to divide thorn into two 

 classes : 



1. Trachytes. 2. Dolerites. 



The Trachytes, or Qreystones, are so called from their rough 

 feel (rpaxur, rough) ; they are generally light-coloured, hence 

 their second name. They are chiefly composed of feldspar, 

 which is rich in silica. 



The Dolerites (SoAcpos, deceptive) are heavier than the 

 trachytes, and are of a darker colour ; they contain less silica, 

 and more of the heavier bases. 

 90 N.E 



THE TRACHYTES, OR FELDdPATHIC LAVAS. 



Trachyte iUelf U fine-grained ; its colour varies from light 

 grey to deep iron-grey ; occasionally it is tinted by the prsstnps 

 of iron. When closely examined it appear* to be a BUM of 

 very minute crystals of feldspar. 



Trachytic Porphyry. The term porphyry, which is of frequent 

 occurrence in every description of primary rooks, is derived 

 from TtofHpvfKoi, purple, which was the colour of an Egyptian 

 mineral to which the name was applied ; bat now any rock is 

 called a porphyry which exhibits large crystals embedded in a 

 compact matrix. Henoe trachytio porphyry is a trachyte 

 through the mass of which crystals of glassy feldspar are dis- 

 seminated. Sometimes this glassy feldspar is not in crystals, 

 but in globules, from the size of a grain of sand to that of a 

 nut ; and these are in a compact mass, without any visible 

 cement to hold them together. From the vitreous or glassy 

 aspect of the mass, the mineral has been termed " pearl-stone." 

 Andetite is the trachyte of the Andes. 

 Clinkstone or Phonolite is a volcanic rock, which 

 exhibits such a tendency to laminate, as to be 

 capable of being used for roofing slate. As 

 its name suggests, it is so compact as to ring 

 when struck with a hammer. It U very probable 

 that clinkstone is trachyte which has cooled 

 under peculiar circumstances. It occasionally 

 contains distinct crystals of feldspar, and is then 

 called " clinkstone porphyry." 



Fig. 24. 



Fig. 23. 



06ndtan or Volcanic Glass 

 is another condition of trachyte, 

 but it has been found that basalt, 

 when melted and rapidly cooled, 

 will produce obsidian ; and hert 

 is another example of the diffi- 

 culty of defining even appa- 

 rently very distinct volcanic 

 rocks, the conditions of their 

 cooling have such an important 

 influence on their ultimate ap- 

 pearance. 



Pumice, the well-known lava, 

 ia this very obsidian, in a cellul :.r 

 form, blown up into its frothy 

 state by the gases or steam it 

 held when in a viscous condition. 



THE DOLERITE8 



have a kindred composition to 

 the trachytes, only their feld- 

 spar is of a kind known as La- 

 bradorite. 



Basalt ia the most prominent 

 member of this group; it is a 

 compact, very dark - coloured 

 rock ; occasionally detached 

 Fig. 27. crystals of augite, olivine, and 



magnetic iron are found in it. 



Its definition, according to Danbeny, is, " an intimate mixture of 

 augite with a zeolitic mineral, which appears to have been formed 

 out of Labradorite by the addition of water, the presence of 

 water being in all zeolites the cause of that bubbling up under 

 the blow-pipe to which they owe their appellation." The Giant's 

 Causeway ia composed of basalt. 



Tufa, or Volcanic Tuff, is the mineralogical term for ashes 

 ejected from volcanic cones ; its grains range from dust, through 

 lapilii (little stones), to breccias, or angular fragments These 

 showers of ashes sometimes collect in vast quantities. Pompeii 

 and Herculanenm were covered by them, and in all volcanic 

 regions they take a prominent position in the strata. They 

 become frequently solidified by pressure, or by the percola- 

 tion of water charged with mineral matter, which acts as a 

 cement. 



We have only noticed the prominent volcanic rocks. 

 They contain many other minerals, which will bo described ir 



THAPPEAN BOCKS. 



These take their name from the fact that occasionally they 

 are found terracing the aides of hills, cutting them into steps, 

 for which word the Swedish is tizppa. Unlike the volcanic 



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