PLAITING OF SCHISTOSE STRATA. 



163 



undulations on a grand scale, represented jfrg-. 269, are not inter- 

 rupted excot by crate'riform ruptures of summits, previously 

 spoken of. 



17. Plaiting t -r folding of schistose strata. Distortions are 

 also observed under other circumstances, in which it seems that 

 beds of a degree of flexibility, or in a pasty condition, have been 

 compressed by two opposing forces, rather than uplifted. Certain 

 facts observed in matter of the structure of schist, naturally lead to 

 this idea. It often happens that the laminae of these deposits, instead 

 of continuing on the same plane, horizontal or inclined, are all found 

 very much contorted without ceasing to be parallel, or folded on 

 themselves into a more or less acute zig-zag (fig. 270). The sup- 

 position as to the mode in which this plaiting has been effected, has 

 been verified by experiments made by Sir James Hall. 



Fig. 270. Contortion of schists. 



Fig. 271. Contortion of coal. 



Entirely similar circumstances occur in coal measures ; all the strata of 

 these deposits, both argilla'ceous and combustible, are found plaited, and 

 often at acute angles (Jig. 271): this is especially remarkable in the coal 

 measures near Mons, in Belgium. 



Now, how did these compressions take place ? In a degree, an explana- 

 tion is required for each locality; but we know that in a deposit of inclined 

 strata, the mass of which is pushed from below upwards, the superior part 

 presses with all its weight on the inferior, and the beds of the latter, being 

 placed between two opposing forces, may fold on themselves, if they are 

 sufficiently flexible. On the other hand, as matters in a state of fusion are 

 "often injected with great force into sedimentary deposits, it is conceived that 

 from this results the lateral compression which produces the same effects. 



18. Origin of Valleys. If mountains are only the result of dis- 

 locations which have taken place on the surface of the globe, by 

 the force of internal agents, there would be no difficulty in account- 

 ing for valleys. The first idea of the origin of valleys was based 

 on excavation by the erosive action of water ; but then mountains 

 having been previously formed, it is clear that water would always 

 follow the natural slope of the soil, and only excavate in that direc- 



17. How is the folding in schistose strata accounted for? 



18. How are valleys produced ? What is meant by vallevs of disloca 

 lion? 



35 



