4.66 Cleavage and Foliation. pam ii. 



sometimes produced by the metamorphosis of homo- 

 geneous fissile rocks ; seeing that foliation and cleavage 

 are so closely analogous in the several above-enumerated 

 respects ; seeing that some fissile and almost homo- 

 geneous rocks show incipient mineralogical changes 

 along the planes of their cleavage, and that other rocks 

 with a fissile structure alternate with, and pass into 

 varieties with a foliated structure, I cannot doubt that 

 in most cases foliation and cleavage are parts of the 

 same process : in cleavage there being only an incipient 

 separation of the constituent minerals ; in foliation a 

 much more complete separation and crystallisation. 



The fact, often referred to in this chapter, of the 

 foliation and the so-called strata in the metamorphic 

 series — that is, the alternating masses of different 

 varieties of gneiss, mica-schist, and hornblende-slate, 

 &c. — being parallel to each other, at first appears quite 

 opposed to the view that the folia have no relation to 

 the planes of original deposition. AVhere the so-called 

 beds are not verv thick and of widelv different minera- 

 logical composition from each other, I do not think that 

 there is any difficulty in supposing that they have 

 originated in an analogous manner with the separate 

 folia. TYe should bear in mind what thick strata, 

 in ordinary sedimentary masses, have obviously been 

 formed by a concretionary process. In a pile of volcanic 

 rocks on the Island of Ascension, there are strata, dif- 

 fering quite as much in appearance as the ordinary 

 varieties of the metamorphic schists, which undoubtedly 

 have been produced, not by successive Sowings of lava, 

 but by internal molecular changes. Near VTonte Video, 

 where the stratification, as it would be called, of the 

 metamorphic series is, in most parts, particularly well 

 developed, being as usual, parallel to the foliation, we 

 have seen that a mass of chloritic schist, netted with 



