394 



REPORT 1856. 



tion (sketched in 1836) in the quarries of Dolbadarn, will illustrate this 

 remark. 



In this case, a and b, which meet each other Fig. 39. 



at a right angle on the plane of the strata, are 

 also perpendicular to that plane, and may be 

 regarded as depending on it, — while c and d, 

 on the contrary, seem to depend on the plane 

 of cleavage, for they are perpendicular to it 

 and to each other, a and b predominate in 

 coarse beds where cleavage is least developed ; 

 c and d in iBne slate ; b is not a joint, but a 

 * band,' or as it is called in Ribblesdale, a ' row ' 

 or small regular fold. 



The joint here marked c constitutes what 

 in the Dolbadarn quarries is sometimes called 

 ' Level bottom ;' and where the ' split' or clea- 

 vage dips from the vertical 4 inches in a yard * ^ 

 to the S.E., the 'level bottom' deviates as 



much from the horizontal to the N.W. The joint marked d makes the 

 'square ends' of the same quarries, from which 'bevel ends' differ by the 

 want of strict perpendicularity with the 'split.' The joint marked a seems 

 to be what is called ' Crub,' — said to ' steal away the level bottom ; ' cl marks 

 undulated lines on the bed formed by the edges of the cleavage. Green 

 veins in this place follow the split-level, and dip here west 42°; parallel to 

 these are the variations of colour — the changes of texture — the boundaries of 

 the workable slate : ' wrinkles' are also parallel to them, being, in fact, dis- 

 continuous small strata, often useful in marking and measuring the effect of 

 a fault. 



§ 10. Occurrence of structures analogous to Cleavage near Greenstone Dykes. 



" A case of this kind fell under the author's notice in 1834-, at Coley Hill 

 near Newcastle*. In the annexed cut rf is a Greenstone dyke, nearly verti- 

 cal, and between 20 and 30 feet across, 

 ranging east and west, and appearing 

 at the surface. 



" s is the ordinary coal shale, which 

 is, as usual, very much laminated at a 

 moderate distance (a few yards) from 

 the dyke, and contains fern leaves and 

 other plants between the laminae. 



" At the sides of the dyke the hori- 

 zontal lamination is obscured, the slaty 

 mass is indurated, and traversed by 



numerous vertical divisional planes parallel to the faces of the dyke, most 

 numerous near the dyke, so as to occur in every half- inch of breadth, but 

 becoming less and less abundant in the parts removed from the dyke till they 

 entirely vanish. On the horizontal section, the lines of these vertical planes 

 would, on a minute scale, represent the cleavage edges of slate." 



Another remarkable case occurred to rne while examining the great green- 

 stone dyke, of Brockhill, in the Abberley district, first described by Murchi- 

 son. This dyke measures 30 feet across ; its structure is rather tabular than 

 prismatic ; it divides the sandstones and marls of the old red series. " For 



* Treatise on Geology, vol. ii. p. 86, first edition (1839). 



Fig. 40. 



