372 



REPORT 1856. 



brittle ; and it is intermixed with schist. This limestone seems to pass by 

 gradation into flinty slate." 



In his 'Introduction to Geology' (published 1813) the same author ex- 

 presses a positive opinion. Speaking of slate, he observes, — " This rock is 

 always represented as stratified ; but in this respect it resembles gneiss and 

 mica-slate, and the slaty and tabular structure 

 are, I conceive, the effect of crystallization, 

 depending on the nature of its constituent 

 parts."— P. 86. 



The earliest notice of a real and firm di- 

 stinction between cleavage and stratification, 

 derived from English examples, which I have 

 met with, is in Otley's ' Concise Description 

 of the English Lakes*.' The modest and 

 intelligent author, speaking of the middle 

 division of the slaty rocks, notices their pre- 

 valent though obscure stratification dipping 

 to the south-east, speaks of the beds of slate 

 with frequently vertical cleavage, and adds, 

 •' but it is found in various digrees of inclina- 

 tion, both with respect to the horizon and planes 

 of stratification." 



In 1821 I made the acquamtance of this able 

 author, verified his remarks on slaty cleavage, 

 and in the same year sketched some of the 

 more curious and special phaenomena in the 

 Lake district, which caught the attention of 

 W. Smith, then engaged on his geological map 

 of that country f. In the mind of that great 

 observer cleavage was separated from strati- 

 fication, and regarded as a kind of crystalliza- 

 tion, running in particular beds. 



Dr. MacCuUoch was too practised in obser- 

 vations among primary rocks not to have ob- 

 served the peculiarites of slate, and we find 

 him distinguishing cleavage from stratification, 

 and referring it to concretionary actionX- 



s s' s" are bands of stratifica- 

 tion, displaced by a small fault/", 

 across which, and across the stra- 

 tification, two small spar veins 

 run quite straight. " The curved 

 lines are edges, more than usually 

 flexuous and symmetrical, of a 

 scaly structure, lying obliquely to 

 the plane of cleavage." (Is this 

 a case of secondary cleavage ?) 



§ 2. Cleavage continuous through large ranges of country. 



Notwithstanding these and probably many other partial views which 

 recognized some diff'erence between cleavage and stratification, it was re- 

 served for Professor Sedgwick, in the year 1835§, to define in a satisfactory 

 manner the essential character of slaty cleavage, and to show its exact place 

 in the series of changes by which soft argillaceous deposits have been stra- 

 tified and solidified, cleft and jointed. Instructed by the repeated examina- 

 tions of the schistose rocks of Westmoreland and Wales (begun in 1822), 

 how to discover the almost evanescent traces of bedding, which in some 

 cases are all that metamorphic action has left, and recognizing in these 



* Keswick, 1823. There was an earlier publication in the Kirkby Lonsdale Magazine, 

 1820. t See Memoir of W. Smith, p. 99. 



t Journal of the Roy. Inst. 1825. System of Geology, 1831, i. 139 ; ii. 186. 

 § Geol. Trans. 2nd series, vol. ii. 



