82 JOINTS IN STRATA. 



In clay, vertical joints are numerous, but small and confused, 

 whereas in indurated shale they are of extraordinary length, very 

 straight, and parallel, dividing the rock into rhomboidal masses. This 

 may be well studied in the shale, which alternates with mountain 

 limestone, at Aldstone Moor in Cumberland. Khomboidal joints are 

 frequent and very regular in coal. 



In limestone the vertical joints are generally regular, and arranged 

 in two sets, which cross at nearly equal distances, and split the beds 

 into equal-sized cuboidal blocks ; and thus the mountain limestone is 

 found to be divided into vast pillars which range in long perpen- 

 dicular scars down the mining dales of the north of England. 



All water-formed rocks, after being upheaved, dry and shrink. 

 The superficial beds in any quarry may be seen to be divided more 

 perfectly and into smaller pieces than the masses which are deeper- 

 seated and moist. This shrinkage is not merely lateral, but to some 

 extent vertical also, and these shrinkage planes are the beginnings of 

 joints. Afterwards, when the strata became strained and bent during 

 the changes of level in land, these planes became extended and sys- 

 tematised in definite and parallel directions. 



In slate districts, the joints, more numerous and more regular 

 than in any other known rock, have almost universally a tendency 

 to intersect one another at acute and obtuse angles, and thus 

 to dissect whole mountains into a multitude of angular solids, 

 with rhomboidal or triangular faces, which strongly impress upon the 

 beholder the notion of an imperfect crystallisation, produced in these 

 argillaceous rocks since their deposition and consolidation by some 

 agency, such as heat or pressure, capable of partially or wholly obliterat- 

 ing the original marks of stratification ; but we may with more proba- 

 bility here also appeal to tension in successively different directions as 

 the true cause of these phenomena. 



Vertical joints are frequent in granite and appear to have definite 

 directions. The trihedral and polyhedral vertical prisms of basalt, 

 and some other igneous rocks, coupled with their regular transverse 

 divisions, seem to give us the extreme effect of regularity in the 

 division of rocks by the process of condensation, from the state of 

 igneous expansion. 



Cause of other Joints and Fissures. That contraction aft 

 partial consolidation of the mass is the general immediate cause o 

 the numerous fissures of rocks, may easily be proved by a variety 

 facts observed in conglomerates, where pebbles, and in other rock 

 organic remains, are split by the joints. According to the circum- 

 stances of the case, this process has produced in basalt, slate, and 

 coal, fissures so regular as to give to the rock a largely crystalline 

 structure, but left in sandstone mere irregular cracks. 



From Mr. Gregory Watt's experiments on fused basalt, and some 

 other notices by different authors, we know that a continued application 

 of even moderate heat to a previously solidified body may be sufficient 

 to develop in it new arrangements of the particles, new crystalline 

 structures, new chemical combinations, and to cause a real transfer of 



