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GENERAL OBSERVATIONS ON JOINTED STRUCTURE. 



through both the surrounding beds, and the large included concretions or ball-stones, 

 enabling the workmen to contend more easily with these hard sub-crystalline masses. 

 It is thus demonstrated, that such joints resulted from one of the last changes super- 

 induced upon the strata after their deposit. As the same method of proof led Professor 

 Sedgwick to show, that the cleavage peculiar to slates is one of the last changes which 

 that description of rocks experienced, it might seem that joints and slaty cleavage are 

 of the same class of phenomena. That author is, however, of opinion, that although 

 they have both been caused by crystalline forces, joints and cleavage planes are to be 

 clearly distinguished from each other, and numerous confirmations of the truth of this 

 observation will be detailed in the subsequent pages, in describing the Silurian rocks of 

 South Wales ; where mountains affected throughout by slaty cleavage are shown to be 

 as distinctly jointed as these now under consideration (see particularly the chapter on 

 Caermarthenshire) . That these different effects may be due to the same cause is ren- 

 dered more intelligible by reference to somewhat similar phenomena. For example, in 

 a six-sided prism of quartz the planes of cleavage are distinct from those of the prism, 

 so that while it is impossible to cleave the crystals parallel to the plane of the prism in 

 the same way as slaty rocks cannot be cleaved parallel to the joints, so the quartz 

 crystal, like the older schists, may be cleaved ad infinitum in the direction of the 

 cleavage planes. Joints, indeed, have one property which alone eminently distin- 

 guishes them from slaty cleavage : the latter often maintains an unity of direction and 

 precision of parallelism, cutting through large regions, independent of the direction 

 or inclination of the strata • while joints according to my observations invariably alter 

 their direction with every change in the axis of elevation. 



Judging from the known crystalline tendencies of various mineral substances, we 

 cannot doubt, that the composition of differently constituted masses has had a very 

 perceptible influence in predetermining symmetrical joints. Thus in all rocks composed 

 of a mixture of lime, clay, and sand, (impure calcareous rocks,) or in argillaceous or 

 sandstone strata, provided they are hard, joints are frequent and neatly defined ; but 

 in the same masses when incoherent, the joints are few, and so irregular in their 

 direction as seldom to be referrible to any given forms. Hence we may infer, that where 

 the strata have obeyed certain chemical laws, the cleavages are perfect, or approach 

 those of crystalline bodies ; but where those laws have acted less perfectly, they are 

 less defined. Should it be contended, that the direction of the joints must have been 

 connected with the elevation of the strata, (as without this hypothesis it is difficult to 

 conceive how the joints could have preserved such symmetrical relations to the. axis of 

 elevation,) it might be replied, that although the directions of the joints vary with the 

 dislocations of the strata, the different divisional planes may be all found to coincide, 

 provided the disturbed strata be readjusted in their original positions. Now some of 

 my observations would lead me to think that rocks of every age, if only hard and of a 

 suitable composition, have a jointed structure the result of crystalline action, and that 



