600 



METAMOKPHIC EOCKS. 



[Ch. XXXVl 



CHAPTER XXXVL 



Origin of the metamorpliic rocks, continued — Definition ot joints — slaty cleavage 

 and foliation — Supposed causes of these structures — Mechanical theory of 

 cleavage — Condensation and elongation of slate rocks by lateral pressure — 

 Supposed combination of crystalline and mechanical forces — Lamination of 

 some volcanic rocks due to motion — Whether the foliation of the crystalline 

 schists be usually parallel with the original planes of stratification — Examples 

 in Norway and Scotland — Foliation in homogeneous rocks may coincide with 

 planes of cleavage, and in uneleaved rocks with those of stratification — Causes 

 of irregularity in the planes of foliation. 



We have already seen that crystalline forces of great intensity have 

 frequently acted upon sedimentary and fossiliferous strata long subse- 

 quently to their consolidation, and we may next inquire whether the 

 component minerals of the altered rocks usually arrange themselves in 

 planes parallel to the original planes of stratification, or whether, after 

 crystallization, they more commonly take up a different position. 



In order to estimate fairly the merits of this question, we must first 

 define what is meant by the terms cleavage and fohation. There are 

 four distinct forms of structure exhibited in rocks, namely, stratification, 

 joints, slaty cleavage, and foliation ; and all these must have different 

 names, even though there be cases where it is impossible, after care- 

 fully studying the appearances, to decide upon the class to which they 

 belong. 



Professor Sedgwick, whose essay " On the Structure of large Mineral 

 Masses" first cleared the way towards a better understanding of this diffi- 

 cult subject, observes, that joints are distinguishable from lines of slaty 

 cleavage in this, that the rock intervening between two joints, has no 

 tendency to cleave in a direction parallel to the planes of the joints ; 

 whereas a rock is capable of indefinite subdivision in the direction of its 

 slaty cleavage. In some cases where the strata are curved, the planes of 

 cleavage are still perfectly parallel. This has been observed in the slate 

 rocks of part of Wales (see fig. 7 06), which consist of a hard greenish 



Fig. T06. 



Parallel planes of cleavage intersecting curved strata. (Sedgwick.) 



slate. The true bedding is there indicated by a number of parallel 

 stripes, some of a lighter and some of a darker color than the general 



