Ch. XXXVL] IKREGULAEITIES IK FOLIATION. 609 



schists in Anglesea was approximately in the planes of bedding ; and 

 Professor Ramsay, in 1841, observed the same in regard to the gneiss 

 and mica-schist of Arran. The last-cited geologist says, in reference to 

 Anglesea, that the metamorphism probably took place when the Lower 

 Silurian volcanos were in activity, and therefore long before the cleavage 

 of the Welsh rocks ; for the cleavage of the latter affects in common the 

 Lower Silurian and the Cambrian strata. In the same memoir he adds, 

 when refei-riug to Mr. Darwin's theory of foliation, " that if the rocks be 

 uncleaved when metamorphism occurs, the foliation planes will be apt 

 to coincide with those of bedding ; but if intense cleavage has preceded, 

 then we may expect that the planes of foliation will lie in the planes of 

 cleavage."* 



From what I have myself seen in the Grampians, both in Forfarshire 

 and Perthshire, I have always concluded that MacCuUoch 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 .xS the intercala- 

 tion of masses of limestone, and of chloritic, actinolitic, and horn- 

 blende 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 car- 

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

 the ingredients composing alternate layers in most sedimentary de- 

 posits, 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 

 crystallized. 



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 raagnesian limestone 

 (see p. 37). 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 current-action, imparted 

 to a deposit from water. Moreover, when we look for stratification in 

 crystalline rocks, we must be on our guard not to expect too much reg- 

 ularity. The occurrence of wedge-shaped masses, such as belong to 

 coarse sand and pebbles, — diagonal lamination (see p. 16), — ripple-maik, 

 — unconformable stratification (p. 61), — the fantastic folds produced by 

 lateral pressure, — faults of various width, — intrusive dikes of trap, — or- 

 ganic bodies of diversified shapes, — and other causes of unevenness in the 

 planes of deposition, both on the small and on the large scale, will inter- 

 fere with parallehsm. If complex and enigmatical appearances di'd not 

 present themselves, it would be a serious objection to the metamorphic 

 theory. 



In the accompanying diagram I have represented carefully the lami- 



* Geol. Quart. Journ. 1853, vol. ix. p. 172. 

 39 



