Ch. XXXYL] lamination of CLAY-SLATE. 757 



blue roofing-slate, while part is extremely quartzose, the whole mass 

 passing downwards into micaceous schist. The vertical section here 

 exhibited is about three 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 sedimentary 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 laminae 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 

 movement, and a similar process may account for what the quarrymen 

 call " the grain " in some granites, or a tendency to split in one direc- 

 tion 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 rea- 

 son 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 metamorphic 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 " primitive." 



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



