746 METAMOKPHIC ROCKS. [Ch. XXXVI. 



newer as the older beds. As an illustration of such partial conver- 

 sion into gneiss of portions of a highly inclined set of beds, I may 

 cite Sir R. Murchison's memoir on the structure of the Alps. Slates 

 provincially termed " flysch" (see above, p. 307), overlying the num- 

 mulitic limestone of Eocene date, and comprising some arenaceous 

 and some calcareous layers, are seen to alternate several times with 

 bands of granitoid rock, answering in character to gneiss.* In this 

 case heat, vapor, or water at a high temperature may have traversed 

 the more permeable beds, and altered them so far as to admit of an 

 internal movement and rearrangement of the molecules, while the 

 adjoining strata did not give passage to the same heated gases or 

 water, or, if so, remained unchanged because they were composed of 

 less fusible or decomposable materials. Whatever hypothesis we 

 adopt, the phenomena establish beyond a doubt the possibility of the 

 development of the metamorphic structure in a tertiary deposit in 

 planes parallel to those of stratification. 



Whether such parallelism be the rule or the exception in gneiss, 

 mica-schist, and other formations of the same family, is a question 

 which I shall discuss at length in the next chapter. 



CHAPTER XXXVI. 



metamorphic rocks, continued. 



Definition of joints, slaty cleavage, and foliation — Supposed causes of these struc- 

 tures — 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 folia- 

 tion of the crystalline schists be usually parallel with the original planes of strati- 

 fication — Examples in Norway and Scotland — Foliation in homogeneous rocks 

 may coincide with planes of cleavage, and in uncleaved rocks with those of 

 stratification— Causes of irregularity in the planes of foliation. 



We have already seen that chemical 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 foliation. There 



* Geol. Quart, Journ., vol. v. p. 211, 1848. 



