ON CLEAVAGE AND fOLIATION IN ROCKS. 



389 



" 1. A compression of their mass in a direction everywhere perpendicular 



to the planes of cleavage. 

 "2. An expansion of their mass along the planes of cleavage in the direc- 

 tion of a line at right angles to the line of incidence of the planes of 

 bedding and cleavage; or in other words, in the direction of the dip 

 of the cleavage. 

 " 3. No proof has been found that the rock has suffered any change in the 

 direction of the strike of the cleavage planes. We must therefore pre- 

 sume that the masses of rock have not been altered in that direction." 

 These conclusions, presented in IS^e, on the sure evidence of the changed 

 forms of shells, trilobitts, &c., were extended in 1848 to slates in which no 

 traces of any organic forms had been observed. The evidence in this case 

 was found by examination of the mechanical structure of the slates, especially 

 by certain apparently brecciated slates including masses of discernible mag- 

 nitude, and distinct colour and quality. Such are frequent in Westmoreland . 

 and Cumberland about Rydal, in Langdale, Patterdale and Borrowdale. 

 " In all these slaty breccias, the included masses are flatter between the 

 planes of cleavage than in any other direction. Their flattest sides are always 

 parallel to the cleavage planes, — they are usually rather longer on the line of 

 dip of the cleavage than along the strike, — thus confirming the opinion that 

 the rocks have expanded in the direction of the dip of the cleavage." 



The Diagram No. 35 represents the 

 appearance of the included masses on Fig. 35. 



the plane of cleavage, they being 

 somewhat elongated in the line of 

 dip ; while Diagram 36 gives the ap- 

 pearance of similar masses on the 

 edge of the same sheet of slate, the 

 fragments being all more or less flat- 

 tened between the planes of cleavage. 

 It is curious to observe in some of 

 these brecciated slates which have 

 undergone much metamorphosis, 

 crystals which have suffered no 

 change by compression. These cry- 

 stals (e. g. garnets) have probably 

 been generated in the mass by the 

 metamorphic actions consequent on 

 communicated heat. 



It is obvious, that with such a 

 structure the easy cleavage of slate 

 in parallel planes is completely pro- 

 vided for. Moreover, in each sheet 

 of slate, where the parts are sensibly 

 extended in the direction of dip, 

 there is a somewhat greater facility of fracture in that direction than in any 

 other. This comparative facility of fracture is called by Mr. Sharpe "se- 

 condary cleavage*;" it is of some importance in the working of slate, and 

 gives rise to the terms " end " (e in fig. 35), and "side" {s in fig. 36). 

 Slates are best split by inserting the tool at the end. 



The labours of Mr. Sorbyf now claim attention. Accustomed to investigate 



* This term is not used in the same sense by other writers, 

 t Edinb. New Phil. Journal, 1853. 



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