GENERAL OBSERVATIONS ON JOINTED STRUCTURE. 



247 



the divergent directions in which the joints are arranged is the consequence of the me- 

 chanical process of elevation. On the other hand other data seem to render it probable 

 that these two causes so distinct in themselves have sometimes cooperated to bring 

 about the different directions of the joints. The latter inference is, indeed, forced upon 

 the mind on examining certain ellipsoids of elevation, — such, for example, as the 

 Wren's Nest at Dudley, in which the direction of the joints changes with every bend in 

 the external folds of the limestone. Many observations, however, infinitely more than 

 those which are offered in the appendix, are required to solve this problem. 



Neither can it perhaps be yet satisfactorily determined, whether the crystalline divi- 

 sions in the mass were influential in inducing lines of elevation to take given directions; 

 or that the great lines of fissure have simply resulted from volcanic or gaseous efforts, 

 acting in different directions according to the greater or lesser resistance offered by the 

 differently accumulated sediments 1 . 



That the crystalline structure was more clearly denned in the more consolidated rock, 

 is evident from the fact, that in soft clay or sandstone (London clay or New Red 

 Sandstone for example) symmetrical joints are rarely if ever discernible ; but when 

 masses of similar composition have been hardened, whether by heat under pressure, or 

 by pressure alone, they then present neatly defined joints. The phenomenon, as re- 

 sulting from heat, has been already adverted to in showing how the Caradoc sandstone 

 has been changed into granular quartz rock, and it will be further developed in the 

 description of the Stiper Stones and of the Lower Lickey, &c. 



Seeing that strata so altered are more affected by joints in the proximity of the vol- 

 canic rock, than at a certain distance from it where they are simply sandstones, and 

 that precisely in proportion as they graduate into an incoherent state the jointed struc- 

 ture disappears, we can I think scarcely avoid inferring, that heat, whether issuing (as 

 in these cases) from an adjacent fissure of eruption, or proceeding on a greater scale 

 from a deeply seated source so as to permeate mountains, has been a prominent agent 

 in the formation of joints. These reflections, as far as my observations go, are ap- 

 plicable to rocks of all ages, and I have merely selected this opportunity to express my 

 opinion, because the formations of the Silurian group expose the phenomena clearly. 

 I shall in future chapters support these views by illustrations drawn from other districts, 

 and in the mean time I refer my readers who cannot travel in Siluria to the illustrative 



1 Professor Phillips supposes " the direction of all lines of disruption to be explicable as a consequence of a 

 general force, applied to masses which in virtue of the divisional planes previously existing in them, presented 

 lines and points of least resistance ; the result modified by considerations as to the weighting of the strata." 

 (Letter to myself.) Mr. Hopkins, if I mistake not, confines his inquiries to the great fissures or faults produced 

 entirely by dynamical forces, which in elevating large tracts have produced cross fractures, &c. ; and though 

 not denying that the jointed structure of many rocks may have been caused by crystalline action, he does not 

 conceive that such a structure could have materially influenced the great lines of dislocation, the direction of 

 which he explains upon mathematical and mechanical principles. (See Cambridge Transactions, vol. vi. p. 1. e t 

 sea., 1835.) 



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