370 



The drawing 





3^ 



^ i 



REPORT 1856. 



§ 1. Cleavage distinct from Stratification. 

 No. 1 i3 a transverse section of the strata in the Snow- 

 ed donian chain from the Menai 

 through the great slate 

 quarries of Mr. Pennant. It 

 shows the argillaceous and 

 arenaceous strata dipping to 

 fr" /<i><:^ /\^z'5 the right (S.E.) or left 



(N.W.), according to the 

 anticlinal and synclinal axes 

 of the district. The fine lines 

 mark the cleavage which 

 crosses the strata, the dotted 

 lines above show the con- 

 tinued arcs of the strata, the 

 deficiencies being attributed 

 to enormous waste of the sur- 

 face ; dotted lines also mark 

 the supposed extension of 

 the cleavage surfaces. The 

 section is an extension of that 

 given by Professor Sedg- 

 wick*, the spectator being 

 supposed to look northward. 

 Professor Sedgwick has 

 also given another section in 

 the same linef, which shows 

 a complete anticlinal at a. 

 I did not observe this with 

 certainty. 



It is remarkable that the 

 investigation of ' cleavage' is 

 one almost entirely British, 

 — till within a very few years 

 almost entirely English ; for 

 neither Saussure, nor Wer- 

 ner, nor any of their follow- 

 ers, appear to have clearly 

 distinguished between strati- 

 fication and cleavage. Saus- 

 sure :[ indeed was too good 

 an observer to pass without 

 record the remarkable lami- 

 nation of the argillaceous 

 and calcareous rocks on the 

 flanks of the great mountains 

 which he so laboriously as- 

 cended. He recognized two 

 sets of fissures, but he attri- 

 buted to stratification the 

 often vertical traces of clea- 

 vage, and was surprised to 



* Geol. Trans. 1835. t Geol. Proc. 1846. : Voyage dans les Alpes, §§ 1049, 1050 (1786). 



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