ON CLEAVAGE AND FOLIATION IN BOCKS. 395 



a space of 30 feet on the north and 17 feet on the south of the dyke, the 

 sandstones and marls are changed in hardness, texture and structure, so that 

 for these breadths they are excavated with the trap ; and from their density, 

 hardness, and resemblance to basalt, amygdaloid or porphyry, may be easily 

 mistaken for primeval rocks of fusion. They have been literally baked under 

 pressure, not roasted with freedom of access and escape for volatile matter." 

 — " In regarding the structures of the stratified rocks, we observe that on 

 approaching toward the dyke the stratification grows less distinct and sud- 

 denly becomes untraceable ; that instead of it, especially on the south side, 

 a great abundance of angularly intersecting dirisional planes occur, so as to 

 produce prismatic structures perpendicular to the plane of the dyke. Further, 

 we observe, parallel to the dyke, to a distance of 30 or more feet from it, 

 several very long, very straight, nearly vertical joints, continuous through all 

 the beds, without any sign of vertical displacement, or any mark of lateral 

 disturbance, unless the appearance of broad striation or narrow fluting, 

 which horizontally marks the vertical sandstone surface, 30 feet from the 

 dyke on the north side, be of the nature of slickenside, and referrible to 

 lateral movement*." 



If these examples be attentively considered, it will appear that under the 

 circumstances described — heat being probably the principal agent, and pres- 

 sure very little if at all evident — the following changes occur, near to and 

 parallel to the heating surfaces : — 



1. Extinction of the stratified structure. 



2. Production of a new structure. 



3. Accon)panied in one case by great molecular and mineral changes. 

 But it must be remarked, that the change indicated in the second of these 



sentences is really distinct from that which slate has undergone. Slate is 

 cleavable in all its parts, more or less perfectly ; because its ultimate mole- 

 cular texture is altered to such a condition ; near these dykes the rocks are 

 cleft indeed, but not further cleavable ; split, but not traversed by numerous 

 planes of easy fissility. 



I have seen phaenomena of a somewhat similar character, but less marked, 

 near great faults, as, for example, in the line of the Craven fault in 

 Yorkshire. 



§11. The Cleat in Coal. 



In the northern coal districts of England, and in other tracts, there exists, 

 besides the lamination parallel to the bounding surfaces of the beds, a series 

 of approximate often nearly vertical divisional surfaces, along which the 

 coal admits of easy fissility. This structure is called cleat, and it is of the 

 greatest importance in coal working, since parallel to it the ' headways' are 

 driven in the ' post and stall ' workings of Northumberland and Durham, and 

 parallel to it the ' banks' are wrought in the ' long wall' and 'board and end' 

 systems of Yorkshire and Derbyshire. Cleat is little aff'ected by fractures, 

 or undulations of the strata. It has usually one persistent course across a 

 large district, — the same direction often obtains in neighbouring districts, and 

 even prevails over the whole of a great carboniferous region, 'rhus in North- 

 umberland and Durham the cleat runs most generally to the north-west (true); 

 its 'strike' is in that direction. The most general strike of the beds is to 

 the N.N.E. The same direction of cleat is prevalent in Yorkshire and 

 Derbyshire, and this whether the beds strike eastward, as near Leeds and 

 Sheffield ; or southward, as near Huddersfield and Chesterfield. The same 

 direction prevails in Lancashire. 



* Memoirs of Geological Survey of Great Britain, vol. ii. pt. 1, p. 156. 



