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grained argillaceous beds, such as slates, shales, or marls, the coarser 

 beds are unaffected by cleavage, while the finer-grained ones are 

 often pervaded by it. Indeed, there appears a strict proportion be- 

 tween the degree of intimate Assuring of the rocks by cleavage and 

 the degree of comminution of the particles. Connected probably 

 with this interruption in the propagation of the cleavage, the author 

 has observed another modification of the cleavage planes, — namely, 

 that they tend to curve a little from the normal direction, in the fine- 

 grained argillaceous beds, approximating to parallelism with the sur- 

 faces of bedding of the adjoining coarser mechanical deposits, as they 

 approach them, showing in a transverse section, a kind of gentle sig- 

 moid flexure. This fact is well illustrated in the cleavage-traversed 

 rocks at the base of the anthracite coal-formation of Pennsylvania, 

 where the red shales alternate with the lower beds of the coal-sus- 

 taining conglomerates and coarse sandstones. These remarkable 

 facts seem sufficient of themselves to refute the hypothesis, some- 

 what in favour at present, of the purely mechanical origin of the 

 cleavage-producing force ; for we cannot conceive how a mechanical 

 force either of compression, or of tension, transmitted, as necessarily 

 it must have been, very equally, through parallel layers of coarse and 

 fine material, should have exerted no Assuring action the moment it 

 reached the surface of the coarser beds, and yet have been able to 

 cleave into thin parallel slaty laminae the whole body of the finer- 

 grained argillaceous strata. One would more naturally suppose that 

 the less finely-aggregated softer mud rocks or shales would have 

 been even less easily fissured into sharp cleavage joints than the 

 more massive and better cemented grits. 

 Foliation. 

 The relations of the foliation or crystalline lamination of meta- 

 morphic strata to the cleavage planes and the planes of stratification, 

 are next dwelt on. Two facts may be stated of foliation, which 

 possess perhaps the constancy of general laws. One of them is, 

 that this structure, as it is seen in gneiss and mica schist, observes, 

 when the strata are not traversed by cleavage, an approximate 

 parallelism with the original bedding. The author of this paper has 

 beheld apparent exceptions to this rule in several localities near 

 Philadelphia and elsewhere in the United States ; and others have 

 been noticed in Europe by Mr D. Sharpe and other good observers. 



