1855.] FORBES — FOLIATION OF ROCKS. IG9' 



We can only regard the truly cleaved rocks as mechanical aggre- 

 gates ; and the more foliated or crystalline such rocks become, the 

 less perfectly do we find the cleavage-planes developed. 



The microscopic researches of Sorby* and others have proved 

 that in slates possessing the most perfect cleavage we have a mass 

 composed of minute rounded grains of mica, decomposed felspar, 

 quartz-sand, phosphate of iron, and other substances, all easily 

 recognizable, and having all the characters of a waterworn deposit, 

 apparently not in any way mineralogically different from the state 

 they were in when originally deposited as a sediment. In fact they 

 are nothing further than a consolidated mud, in which a lamellar 

 structure has been induced by mechanical action ; and the synthetical 

 experiments of Sorby on this subject give the most conclusive evidence 

 as to the mechanical nature of cleavage ! 



Admitting that cleavage and foliation were identical, how incon- 

 sistent with the parallel structure of cleaved rocks should we find the 

 numerous cases of complicated and contorted foliated structure, which 

 are so common, and where it is almost impossible to believe that 

 these contortions have arisen from any twisting of the lines, either 

 of stratification or cleavage, as the mechanical forces then brought 

 into play would be so compound as hardly to be conceivable. 



It must, nevertheless, be admitted, that the lines of foliation and 

 the planes of cleavage do often agree and are parallel to one another ; 

 and several opportunities of confirming this have come under my 

 observation. 



(Case 3.) — In Espedalen, Norway, the foliation of the mica- 

 schists and hornblende-gneiss is parallel to the cleavage of the clay- 

 slates of the district, both running nearly E. and W., and dipping 

 N. at various angles from 10° to 50°. The line of bedding cannot be 

 determined, but seems probably to be in the line of cleavage. 



(Case 4.) — On the side of Loch Lomond, at Luss, I observed 

 that the clay-slate there quarried has a cleavage-strike of N.E.- 

 S.W., and dip of 70° S. ; the stratification being uncertain. A 

 little further north the mica-schist appeared (the points of contact 

 were hidden by the soil), and the strike was found to be 65° N.E.- 

 S.W., with a dip of 60° to S. ; which may be regarded as nearly 

 coinciding. 



(Case 5.) — In the slate-quarries at Luss we have also instances of 

 the cleavage-lines being bent by coming in contact with quartz-veins, 

 just as described by Mr. Sharpef; but I found that, where these 

 veins occurred, not only were the ends of the cleavage-planes bent, 

 but around the quartz-vein there was developed a distinct curved 

 foliation, induced by a deep-green chlorite, with here and there a 

 little mica. This foliation sometimes extends for a short distance 

 into both the slate and the quartz, but appears quite independent of 

 the cleavage, though with an evident relation to the curves of the 

 veins themselves, which are very irregular. The quartz itself is 



* Edin. Pliil. Journal, July 1853. 



t Quart. Journ. Geol. Soc. vol. v. p. 11". 



