122 



CLEAVAGE, JOINTS, AND STRATIFICATION 



Upper 

 Gallery. 



The remainder of the specimens in this case more especially 

 illustrate some of the relations of cleavage, joints, and stra- 

 Wall-case 46. tif cation. Cleavage may be defined to be a re-arrangement 



of the particles and larger substances that enter into the 

 composition of certain rocks so as to produce a fissile 

 structure or a tendency to split in given directions, some- 

 times accidentally coincident or nearly coincident with 

 planes of stratification, but generally transverse to these at 

 every possible angle. The origin of cleavage has been 

 referred to " electric action " and to " polar " and " crys- 

 talline forces." These words, however, convey but little 

 definite impression to the mind when viewed in connexion 

 with cleavage, and from the writings of Professor Phillips 

 and Mr. Daniel Sharpe, but especially by the memoirs and 

 experiments of Mr. Sorby and Professor Tyndal it now 

 begins to be generally understood that cleavage is the result 

 of pressure induced by the disturbances that have fre- 

 quently produced contortion of cleaved strata. It is not 

 the case, however that all contorted strata are cleaved, 



Probably all 

 the portions of strata intensely cleaved have been buried 



deep beneath superincumbent masses when those forces 

 operated that produced cleavage, whereas uncleaved con- 

 torted rocks were generally so near the surface that they 

 were able more easily to fracture and yield, so that their 

 component particles were not forced to readjust themselves 

 so as to produce cleavage. It is not necessary here to discuss 

 the causes of these disturbances. It is sufficient to recog- 



the fact that highly contorted rocks have been subject to 

 pressure (generally lateral) and that " this force by changing 

 the dimensions of the rock has so re-arranged the laminar 

 particles as to cause a very great majority to lie in a plane 

 perpendicular to it" — (Sorby.) If these rocks contain water 

 in minute cavities, these cavities would necessarily be elon- 

 gated in the same direction. The whole rocky substance has, 



although most cleaved rocks are contorted. 



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