183 



§ 3. Slaty Structure, Cleavage, S^c. 



The subjects introduced in this section of the paper are described 

 in considerable detail. The author first compares the structure of the 

 great Cumbrian zone, of green slate and porphyry, with the structure 

 of the principal chain of North Wales, and shows their perfect ana- 

 logy. In one respect, however, the two regions are remarkably con- 

 trasted. The Cumbrian system has few contortions or undulations, 

 probably in consequence of the great abundance of alternating beds 

 of porphyry ; whereas a transverse section through the Welsh chain, 

 exhibits a continued series of longitudinal anticlinal and synclinal 

 lines. He also compares the structure of an upper slaty series in 

 Westmoreland and Lancashire, with a corresponding upper series in 

 North and South Wales, pointing out the circumstances in which 

 they agree and in which they difter. 



In all these regions occur many beds with a slaty cleavage, which 

 is entirely distinguished from a jointed structure by its indefinite sub- 

 division, and it is never found to coincide vnth the true plane of strati- 

 fication. These planes of stratification and cleavage sometimes dip 

 to the same point, and sometimes to opposite points of the compass ; 

 they are stated to be inclined to each other, sometimes at an angle 

 less than 10°j on the average at an angle of 30° or 40°, and in no 

 instance at 90°. Where the slaty structure is well developed, the 

 strike of the cleavage planes coincides nearly with the strike of the 

 beds J and this important rule holds true in countries where the beds 

 themselves are thrown into a series of anticlinal and synclinal planes. 

 The author adds, that there are regions in North and South Wales, 

 thirty miles in extent, and many miles in breadth, where the cleavage 

 planes (notwithstanding the numberless contortions of the beds) pre- 

 serve an undeviating direction and dip. He states that in many large 

 slate quarries there is no indication whatsoever of the true bedding ; 

 but whenever the slates have a striped structure, the stripes (so well 

 known in the Cumberland and Welsh roofing slates) are parallel to 

 the true beds. To this rule there is no exception in the regions de- 

 scribed ; and in thousands of instances the stripes are seen to be 

 parallel to the alternating masses of 'coarse greywacke', and to cal- 

 careous beds with organic remains. 



The author then describes a flaggy, passing into a finely laminated, 

 structure, parallel to the bedding. He points out the manner of di- 

 stinguishing this from a true slaty structure, which may readily be 

 done in a quarry, and, generally, even in examining hand specimens. 



In this view, a laminated structure and a slaty structure differ so 

 entirely in their origin, that however nearly they may resemble each 

 other in some instances, they ought never to be confounded under 

 the same name. 



Finally, he distinguishes cleavage planes from the contorted laminae 

 of argillaceous schists ; and endeavours to prove, by a long series of 

 sections derived from various parts of North and South Wales, that 

 the introduction of a crystalline cleavage was the last chemical change 

 superinduced on the slaty deposits before they became entirely solid. 



He then speculates on the enormous amount of force necessary to 



