THE STRUCTURE AND CLASSIFICATION OF ROCKS. 



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the coarser beds and the finer, the twisted and the straight, have all been subjected to 

 one change. Crystalline forces have re-arranged whole masses of them, producing a 

 beautiful crystalline cleavage, passing alike through all the strata. And again, through 

 all this region, whatever be the contortions of the rocks, the planes of cleavage pass on 

 generally without deviation, running in parallel lines from one end to the other, and 

 inclining, at a great angle, to a point only a few degrees west of the magnetic north. 

 Without considering the crystalline flakes along the planes of cleavage, which prove that 

 crystalline action has modified the whole mass, we may affirm that no retreat of parts, no 

 contraction of dimensions, in passing to a solid state, can explain phenomena such as 

 these. They appear to me only resolvable, on the supposition that crystalline or polar 

 forces acted on the whole mass simultaneously, in given directions, and with adequate 

 power. It is not, however, necessary to suppose that these effects were produced in a 

 short lapse of time. In speculating on the time required for the completion of these 

 phenomena, we are free from any unnecessary limitation." Whether this points to the 

 true solution of an obscure problem, or not, it is evident that the structures indicated by 

 joints and cleavage were impressed upon the rocks they characterise by agencies acting 

 after their deposition ; for, in several newer fossiliferous strata, symmetrical joints pass 

 through the organic remains imbedded in them. 



The manner in which the two great classes of rocks, the stratified and the unstratified, 

 are disposed in relation to each other, now requires a notice. It is very clear that they 

 have been formed under the operation of totally distinct causes. The parallel arrange 

 ment of the strata, the pebbles of pre-existing rocks and organic remains imbedded in 

 them, together with the ripple-mark, the footprints and other traces of animals which they 

 exhibit, demonstrate their deposition in water ; while the clear marks of having crystallised 

 in cooling from a state of fusion borne by the unstratified rocks, the close analogy of 

 their composition to modern lavas, and often complete identity, plainly prove their 

 igneous origin. In considering the association of the two classes, the most decisive evi 

 dence appears, that while the unstratified masses are uniformly found beneath the stra 

 tified, they have been violently propelled among the strata in a melted state, by the 

 expansive property of an immensely high temperature acting under pressure, occasioning 

 the different inclinations, contortions, and fractures previously described. It is obvious 

 that the materials of the sedimentary rocks must have been deposited horizontally ; yet 

 nothing is more common than to see beds of sandstone and conglomerate in a vertical 

 position. The leaves of ferns in the shale of coal countries, which are often spread out as 

 regularly as dried plants between the sheets of paper in the herbarium of a botanist, are 

 also frequently found highly inclined, and sometimes vertical, though they must have been 

 horizontally disposed when deposited. Fig. 33 represents a common instance of the 



manner in which the unstratified and stratified rocks are associated, in which strata of 

 limestone appear lying upon mountainous masses of granite. At a a the granite has 

 burst through the limestone ; at b it overlies it, as if it had flowed over ; at c it inter- 

 stratifies it, having introduced itself in a rent or fissure produced by the catastrophe of 



