LESSONS IN GEOLOGY. 



gnoisa frequently exhibit*. But tho layers in gneiss are too 



Mllrl to admit of their origin Wing duo to tho do- 



s, wo have an instanco win-: 

 i pnidnrrd from clay-Hlato. There is, in tin- 

 urhood of Dublin Bay, a range of granite hills, omo 



.:. This vast maaa of granite was erupted t 

 a series of clay-slates. These slates have a dull, earthy texture, 

 ami are dark-coloured. According to Professor Jukes, "When- 

 < granite conies to the surface, a belt of slates surround- 

 in.' it 1 into mica-schist, with, in some few places, 

 beds of iss. Crystals of garnet, schorl, andalusite, 

 iko their appearance in these altered slates in greater 

 and gri ater abundance as they approach tho granite." 



At a distance as much as two miles from tho outcrop of the 

 nuiito, tho slates acquire a glaze, or a micaceous lustre, which 

 does not desert tho particles even when the rock is < 

 This micaceous tendency becomes more definite as the granite 

 is approached, until folia and plates of mica are apparent. 



Hero, then, we have a plain case in which clay-slates have 

 become metamorphosed into mica-schist and gneiss. 



A similar instance and, in one souse, more instructive is 

 quoted by Sir Charles Lyell, as being exhibited on the west sido 

 of tho fiord of Christiania, in Norway. Here the granite pro- 

 trudes into fossiliferous strata ; at a distance of 400 yards from 

 tho outcrop of the igneous rock tho stratified rocks are "altered," 

 and begin to exhibit all the appearances of metamorphic rocks, 

 and to contain crystals in their moss of minerals usually classed 

 with igneous rocks. Tho fossils can " rarely bo detected," and 

 in those places where the crystallising force has proceeded to 

 any extent, all traces of organic remains are quite obliterated. 



In Shetland tho same fact is illustrated. There, clay-slates 

 which are in contact with granite become gradually more sili- 

 ceous, and finally merge into hornblende-schist. 



From these and similar facts we conclude that gneiss, and its 

 kindred rocks, are " altered " stratified rocks, altered by the 

 agency of heat. The greatest amount of mctamorphism would 

 be effected where the heat was so intense as to completely fuse 

 tho rock ; in this case an igneous rock would be the result, and 

 therefore it is possible that some igneous rocks, and even 

 kTanite, may have been once stratified rocks which had under- 

 gone fusion. This opinion is received by many geologists, who 

 argue that the presence of phosphates in minerals found in 

 these igneous rocks may be duo to organic remains which have 

 been utterly obliterated, and yet the phosphate of lime found in 

 their bony structures mingled itself with the composition of 

 the molten matter, and there produced such minerals as apatite, 

 which contain phosphorus in a state of combination. 



Gneiss may be said to be schistose granite ; that is, a rock 

 containing the constituents of granite, and yet exhibiting a de- 

 gree of foliation and a disposition to split up into lamina. 

 \Vhen this facility of division increases, the term " schist " is 

 always applied to the rock, and the most prominent constituent 

 of the rock placed before tho word, so as to render the name 

 descriptive. Thus nuca-sckist is a fissile mass of mica and 

 quartz, frequently containing garnets and crystals of hornblende. 

 If talc be in the place of the mica, it is then termed talc-schist; 

 if the material be composed of chlorite and quartz, a chlorite- 

 s:hist is the result ; and thus the schistose rocks are a large 

 class, all probably dependent for their different composition to 

 the circumstances under which they were formed. 



Metamorphosed Calcareous Rocks. This class presents unusual 

 interest, for to this action we owe the existence of that beautiful 

 stone which has been used in every civilised age for ornament 

 and sculpture. Marble is an " altered " or " metamorphosed " 

 limestone. In a preceding lesson, we entered into the formation 

 of limestone and chalk rocks in general, showing that their 

 origin was most probably due to the agency of life ; in fact, they 

 were built up by the united effort of an inconceivable number of 

 minute polyps, and other animalcules. No wonder, then, that 

 snch rocks contain innumerable evidences of organic structure ; 

 and yet no trace of such structure has ever been discovered in 

 marble. The very finest marble in the world, that of Carrara, 

 offers the best illustration of this subject. It abounds in the 

 Massa Carrara, or Apuan Alps, mountains some 0,000 feet high. 

 In the neighbourhood, where the rocks are not altered by meta- 

 morphic action, they are composed of the ordinary Apennine 

 limestone, which belongs to the oolite period, resting on argilla- 

 ceous and siliceous sandstones. Tho limestones are full of fos^ 



ils and flint nodule* ; between them and the sandstones, which 

 are also foBsilif erous, though not so plentifully as the limestone*, 

 is a band of shale. In the neighbourhood of the MAMA Carrara 

 these strata hare been invaded by plutonio rooks, and erase- 

 quently metamorphosed ; the fosiiliferous limestone has become 

 Carrara marble, which doe* exhibit a trace of a fossil ; the 

 flint nodules are no longer found, but in their place crystals of 

 quartz. In some places the lines of stratification can be traced, 

 though this is the exception rather than the rale. The shale* 

 hare been altered into talc-schists, jasper, and hornstone. whilst 

 in the place of the underlying sandstones are quartette and 

 gneiss. This and other similar instances leave no room for 

 doubt that marbles and crystallised limestones owe their charac- 

 ters to the action of heat. It seems not improbable that th* 

 black lines in marble are duo to carbon in a very finely divided 

 state, and that this carbon may be the remains of some vege- 

 table or animal deposit. 



Tho above results were approached by actual experiment 

 by Sir James Hall, who found that chalk, when heated and so 

 pressed as to prevent the escape of the carbonic acid gas, 

 assumed a crystalline appearance. 



Ifctamorpliosed Ar<jillaci ><>.s Kocla. These rocks were origi- 

 nally deposited as clay or Hilt, which was probably derived from 

 the further disintegration of gneiss, and which af torward* under- 

 went metamorphic action. They are now known by the blue 

 and purplish roofing-slates so much used, for which tho aerie* is 

 very valuable. These rocks aro of great thickness, and exhibit 

 the phenomenon of " cleavage " very beautifully. The plane* 

 of cleavage do not run parallel to those of stratification, but 

 almost at right angles to them. The origin of cleavage is not 

 known. Some geologists ascribe it to the action of heat, others 

 to pressure, others to electricity. Very probably all these causes 

 have conspired to produce this curious effect. 



Metamorphosed Arenaceous Rocks. Quartette or Quartz Rock 

 is the sandstone of the group. As wo have said, sandstone i* 

 an aggregation of particles of quartz. When snch a rock is sub- 

 mitted to tho action of great heat, these particles agglomerate, 

 forming qnartzite. This rock is not to be confounded with 

 vein-quartz, which occurs as a white, flinty mass, often in largo 

 quantities. The matrix of the Australian gold is this species 

 of quartz. The difference between the two kinds of quartz is 

 this : quartzite ii an altered bed of sandstone of the same age 

 as the beds in which it is found ; whereas vein-quartx is a 

 deposition in a vein or fissure, which, of course, must have 

 been mode subsequent to the deposition and solidification of 

 the rock. 



The metamorphic systems, as we have seen, are very produc- 

 tive in a mercantile point of view. Marbles, slates, serpentines, 

 quartz ; rock which is used, when ground down, for pottery pur- 

 poses; metallic veins of copper, lead, tin, frequently traverse 

 the beds of the system. Gold and silver, and many precious 

 stones, are among the valuables its beds contain. 



But one of its products demands a longer notice. Plumbago, 

 or black lead, as it is erroneously called, is only found in rocks 

 which are members of this group. Plumbago ia one of the 

 forms of carbon. This element appears in three states: as 

 coal, plumbago, and diamond. The latter ia the purest, and is 

 probably only carbon which has had, under peculiar circum- 

 stances, the opportunity of crystallising. Plumbago contains 

 about four per cent, of iron, the rest of it is pure carbon. 

 There is reason to believe that it is coal which has been meta- 

 morphosed. Sir Charles Lyell gives a typical example of this 

 process. 



At Worcester, in the State of Massachusetts, there occurs a 

 bed of impure anthracite and plumbago, interstratified with 

 mica-schist. Anthracite is heavier than common coal, and more 

 completely mineralised. It has an iridescent lustre, and does 

 not soil tho fingers. From microscopical examination, and other 

 means, there is no doubt that anthracite ia coal which has been 

 somewhat altered ; a step further, and plumbago ia the result 

 At the place indicated, the anthracite ia dug out for fuel, 

 and tho plumbago for pencils. Sir Charles says : " After tra- 

 versing the country in various directions, I came to the conclu- 

 sion that tho carboniferous shales or slates, with anthracite and 

 plants, which at Khode Island often pass into mica-schist, have 

 at Worcester assumed a perfectly crystalline and metamorphic 

 texture, the anthracite having been nearly transmuted into thai 

 state of pure carbon which ia called plumbago, or graphite." 



