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PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



[May 6, 



laminar trachytes, or to some little-understood force of polarity, it 

 appears to me far from superfluous to describe certain facts very 

 completely establishing the sedimentary and metamorphic origin of 

 the schistose rocks of the localities mentioned in the sequel. That 

 the mere repetition of layers of different mineral character is of 

 itself no absolute proof of sedimentary formation is sufficiently 

 apparent from what is often seen in well-known igneous rocks ; but 

 mica-schist sometimes possesses other peculiarities, to which I beg- 

 to call attention. 



In very many stratified rocks the ripple-marks are extremely 

 characteristic of deposition under the influence of a current ; but, if 

 these were found in bent and contorted schists, it would, I think, 

 be rash to base any important conclusion on them, since mere 

 mechanical bendings might so readily mislead an observer. There 

 is, however, a structure generated when ripples are formed whilst 

 material is being deposited and permanently accumulated, which 

 has such well-defined peculiarities that it appears to me almost 

 impossible to confound it with any structure produced by other 

 means. I here allude to what, in the various papers I have pub- 

 lished on the subject*, I have called " ripple-drift," the most im- 

 portant characters of which will be better understood from the fol- 

 lowing drawing : — 



Fig. 1. — Tliin bed of "Ripple- drift " in unaltered sandstone (nat.size). 



Frequently there are quite horizontal beds below and above 

 (a b, cd), and between are beds inclined to them at an angle a dc y 

 which are themselves made up of smaller beds or stratula dipping in 

 the opposite direction, namely from b towards c. Such bands often 

 occur in the green slates of Langdale in Westmoreland, the cleavage 

 cutting them at a considerable angle ; and when the slates arc 

 ground smooth and slightly varnished, the peculiar structure may 



* Eilin. New Phil. Jburn., new ser. vols. iii. p. 112, iv. p. 317, v. p. 275, 

 rii. p. 220 ; ' Geologist,' 1859, p. 137. 



