CLEAVAGE. 



P<i/fo*on of Strata by Master Joi 



slines, backs, lords, &c., are perfectly well known to the workmen, as 

 well as some other very important yet less certain and continuous 

 fissures passing nearly east-north-east and west-south-west. It is 

 according to such joints that the experienced collier arranges his 

 workings, and the slater and quarryman conduct their excavations. 

 Now, surely nothing can be more certain than the inference that some 

 very general and long-continued agency, such as gradual tension or 

 straining, pervading at once the whole mass of these dissimilar and 

 successively deposited strata, was concerned in producing this remark- 

 able constancy of direction in the fissures which divide them all. The 

 deficiency of recorded observations prevents a general development of 

 this important subject by reference to other districts, but it is obvious 

 that a great principle in the construction of the earth is here indicated, 

 which must eventually have an important influence on geological 

 theory. In the meantime, we may remark, first, that these prevalent 

 directions of north by west and east-north-east are those of the prin- 

 cipal mineral veins in the north of England, and that they are also 

 admitted to be very prevalent in the southern and western mining 

 countries as cross-courses; secondly, that these directions are wholly 

 uninfluenced either by the inclination of the strata or by the numerous 

 dislocations to which they are liable. Whatever be the direction of 



the dip, how frequent soever the 

 faults, the lines of the great 

 joints are the same. These lines 

 are frequently the cause of parti- 

 cular courses in rivers and springs, 

 long scars on mountain-sides, and 

 subterranean channels for water. 

 Faults, and dykes, and mineral 

 veins very frequently pass along 

 them, and there is little doubt 

 that the diligent study of them 



will be found to throw much light on some of the most interesting 

 phenomena of geology. 



Cleavage. There is yet another structure not common to all rocks, 

 nor confined to a given geological age. Though most frequently 

 manifested among the Primary strata, it is sometimes observable in 

 others of a later date. This structure is called cleavage, and it 

 consists in a peculiar fissility of the rocks which are affected by it, 

 parallel to a certain plane, which almost always cuts at a considerable 

 angle the plane or curved surfaces of the stratification. In fig. 36, 

 which represents a mass of rocks in which this definite quality of 

 splitting is developed, B B is the surface (curved in this instance) of 

 one bed of the stratification j j is on the plane, here supposed ver- 

 tical, of a joint ; c is one of the planes of cleavage, cutting the sur- 

 face of stratification B B in s-s. Parallel to this plane c, the mass of 

 rock here represented is cleavable by art, and is often actually cleft by 

 nature, into very thin and numerous plates, which, when of suitable? 

 quality and reduced to proper size, constitute the roofing-slates of our 





