PART I. CHAPTER X. 



137 



Slaty Cleavage. 



Fig. 123. 



This disposition of the 

 layers is illustrated in the 

 accompanying diagram, in 

 which I have represented 

 carefully the stratification 

 of a coarse argillaceous 

 schist, which I examined in 

 the Pyrenees, part of which 

 approaches in character to 

 a green and blue roofing 



Lamination of clay-slate, Montagne de Seguinat, slatC, whilc part is extremely 

 near Gavarnie, in the Pyrenees. quartzOSC, the wholc maSS 



passing downwards into micaceous schist. The vertical section 

 here exhibited is about three feet in height, and the layers are 

 sometimes so thin that fifty may be counted in the thickness of 

 an inch. Some of them consist of pure quartz. 



The inference drawn from the phenomena above described, 

 in favour of the aqueous origin of clay-slate and other crys- 

 talline strata, is greatly strengthened by the fact that many of 

 these metamorphic rocks occasionally alternate with, and some- 

 times pass, by intermediate gradations, into rockl of a decidedly 

 mechanical origin, and exhibiting traces of organic remains. 

 The fossiliferous formations, moreover, into which this passage 

 is effected, are by no means invariably of the same age nor of 

 the highest antiquity, as will be afterwards explained. (See 

 Part II.) 



Stratification of the metamorphic rocks distinct from cleav- 

 age. — The beds into which gneiss, mica-schist, and hypogene 

 limestone divide, exhibit most commonly, like ordinary strata, a 

 want of perfect geometrical parallelism. For this reason, there- 

 fore, in addition to the alternate recurrence of layers of distinct 

 materials, the stratified arrangement of the crystalline rocks 

 cannot be explained away by supposing it to be simply a divi- 

 sional structure like that to which we owe some of the slates 

 used for writing and roofing. Slaty cleavage, as it has been 

 called, has in many cases been produced by the regular depo- 

 sition of thin plates of fine sediment one upon another; but 

 there are many instances where it is decidedly unconnected with 

 such a mode of origin, and where it is not even confined to the 

 aqueous formations. Some kinds of trap, for example, as clink- 

 stone, split into laminae, and are used for roofing. 



There are, says Professor Sedgwick, three distinct forms of 

 structure exhibited in certain rocks throughout large districts : 

 viz. — First, stratification; secondly, joints; and thirdly, slaty 

 cleavage ; the two last having no connection with true bedding, 



