756 



IRREGULARITIES IN FOLIATION. 



[Ch. XXXVI. 



From what I have myself seen in the Grampians, both in Forfar- 

 shire and Perthshire, I have always concluded that MacCnlloch was 

 correct in the opinion that gneiss and mica-schist may be considered 

 as stratified rocks, and that certain beds of pure quartz, one or two 

 feet thick, which run for miles in the strike of their foliation, as well 

 as the intercalation of masses of limestone, and of chloritic, actinolitic, 

 and hornblende schists, all indicate the planes of original stratification. 

 At the same time, I fully admit that the alternate layers of quartz, or 

 of mica and quartz, of felspar, or of mica and felspar, or of carbonate 

 of lime, are more distinct, in certain metamorphic rocks, than the 

 ingredients composing alternate layers in most sedimentary deposits, 

 so that similar particles must be supposed to have exerted a molecular 

 attraction for each other, and to have congregated together in layers 

 more distinct in mineral composition than before they were crystal- 

 lized. 



We have seen how much the original planes of stratification may 

 be interfered with or even obliterated by concretionary action in 

 deposits still retaining their fossils, as in the case of the magnesian 

 limestone (see p. Si). Hence we must expect to be frequently baffled 

 when we attempt to decide whether the foliation does or does not 

 accord with that arrangement which gravitation, combined with cur- 

 rent-action, imparted to a deposit from Avater. Moreover, when we 

 look for stratification in crystalline rocks, we must be on our guard 

 not to expect too much regularity. The occurrence of wedge-shaped 

 masses, such as belong to coarse sand and pebbles — diagonal lami- 

 nation (see p. 16) — ripple-marked — unconformable stratification (p. 

 16), the fantastic folds produced by lateral pressure — faults of various 

 width — intrusive dikes of trap — organic bodies of diversified shapes 

 — and other causes of unevenness in the planes of deposition, both on 

 the small and on the large scale, will interfere with parallelism. If 

 complex and enigmatical appearances did not present themselves, it 

 would be a serious objection to the metamorphic theory. 



Mr. Sorby has shown that the 

 peculiar structure belonging to 

 ripple-marked sands, or that which 

 is generated '. when ripples are 

 formed during the deposition of 

 the materials, is distinctly recog- 

 nizable in many varieties of mica- 

 schists in Scotland.* 



In the accompanying diagram I 

 have represented carefully the lami- 

 nation of a coarse argillaceous 

 schist which I examined in 1830 

 in the Pyrenees. In part it approaches in character to a green and 



Fig. 761. 



Lamination of clay-slate, Montagne de Segui- 

 nat, near Gavamie, in the Pyrenees. 



* H. C. Sorby, Geol. Quart. Journ., vol. xix. p. 401. 



