610 



LAMINATION OF " CLAY-SLATE." [Ch. XXXVI 



Lamination of clay-slate, Montagne de Seguinat, 

 near Gavarnie, in the Pyrenees. 



nation of a coarse argillaceous Fig- T09. 



schist which I examined in 1830 

 in the Pyrenees. In part it ap- 

 proaches* in character to a green 

 and blue roofing-slate, while part 

 is extremely quartzose, the whole 

 mass passing downwards into mi- 

 caceous schist. The vertical sec- 

 tion here exhibited is about 3 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. 



There is a resemblance in such cases to the diagonal lamination which 

 we see in sedimentary rocks, even though the layers of quartz and of 

 mica, or of felspar and other minerals, may be more distinct in alternating 

 folia than they were originally. 



M. Elie de Beaumont, while he regards the greater part of the gneiss 

 and mica-schist of the Alps as sedimentaiy strata altered by plutonic 

 action, still conceives that some of the Alpine gneiss may have been 

 erupted, or, in other words, may be granite drawn out into parallel laminse 

 in the manner of trachyte as above alluded to.* 



If the mass were squeezed and elongated in a certain direction after 

 crystals of mica, talc, or other scaly minerals were developed, these may 

 perhaps have arranged themselves in planes parallel to those of move- 

 ment, and a similar process may account for what the quarrymen call 

 " the grain" in some granites, or a tendency to split in one direction 

 more freely than in another. But, as a general rule, the fusion of the 

 crystalline schists does not appear to have gone so far as to allow of 

 motion analogous to that of lava or granite, and for this reason rocks of 

 this class do not send veins into surrounding rocks. In the next chapter 

 we may inquire at how many distinct periods the hypogene or metamor- 

 phic schists can be proved to have originated, and why for so long a time 

 the earlier geologists regarded them as entitled to the name of " primi- 

 tive." 



* Bulletin Soc. Geol. de France, 2e ser. vol. iv. p. 1301. 



