Sect. vL] 



GEOLOGY. 



175 



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displacement which the surroundin 



"late-rocks have 



undergone/' 



Observations on cleavage, to be useful. 



must be numerous and very accurately made. 



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he foliation of the metamorphic schists, that is, tr.e 

 origin of the layers of quartz, mica, feldspar, and other 

 minerals, of which gneiss, micaceous, chloritic, and horn- 

 blendic schists are composed, is intimately connected with 

 the cleavage of homogeneous slaty rocks. Nearly all the 

 proposed observations on cleavage are applicable to folia- 



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tion. Wherever large districts of foliated and ordinary 

 slaty rocks unite, observations would be most desirable. 

 These foliated rocks have all undergone metamorphic 

 action, that is, they have been mineralogicHliy altered and 

 rendered crystalline by chemical attraction, aided by heat ; 

 but this is a most obscure suliject, one on which it would 

 appear that much further light will not be thrown without 

 the aid of a profound knowledge of mineralogy or che- 

 mistry. It is now known that granitic rocks, which have 

 been fluidified (as may be told by their sending great 

 veins into, and including fragments of, the overlying rocks), 



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are foliated in a more or less perfect degree: in these 

 cases the relation of the planes of foliation with those of 

 the adjoining rocks, which have been metamorphosed but 



not fluidified, would be eminently curious. 



the Sea-bottom. — As every sedimentary 



Nature of 



stratum has once existed as the bed of the sea or of a lake, 

 the importance of observations on this head is obvious ; 



* With respect to further observations on this important point, Mr. 

 Hopkins remarks, in his paper ' On the Internal Pressure of Sock 

 Masses ' (Cambridge Philosoph. Transact., vol. viii.), that " the observer 

 should direct his attention especially to those cases in which the inclina- 

 tion of the cleavage planes to the bedding is either small, or nearly 4.5-." 



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