I 



MASTER-JOINTS. 83 



some of the ingredients from one part of the mass to another. From 

 many independent facts it is inferred, as a matter of certainty, that all 

 the strata have locally, and the lower ones perhaps universally, 

 sustained the action of considerable heat since their first deposition, 

 consequent on folding and pressure : we seem, therefore, to be pos- 

 sessed of the clue which is eventually to conduct us to a knowledge 

 of the cause of the different structures observable in rocks independent 

 of their stratification. 



But, though heat be taken as the leading cause of many of these 

 effects, it is by no means inconsistent to suppose that some other 

 independent agent as, for example, electricity might be concerned 

 in modifying the result. From discoveries in electricity, it appears 

 certain that this universal agent is excited in every case of dis- 

 turbance of the chemical or mechanical equilibrium of natural bodies.; 

 and it is especially and very sensibly excited by unequal distribution 

 of heat. Professor Sedgwick's suggestion with reference to Mr. Fox's 

 electro-magnetic experiments on the mineral veins of Cornwall, that 

 electricity was probably concerned in the original production of those 

 veins along which it now circulates, may be perhaps extended to the 

 contents of the joints of rocks ; in the study of which Professor 

 Phillips found abundant reason to believe that the theory of the 

 production of mineral veins is inseparable from that of the joints 

 and fissures, in some of which the metallic substances are deposited. 



The joints in igneous rocks like those in aqueous rocks are due to 

 contraction, but it is contraction on cooling. In granite, the joints 

 are remarkably regular, and generally correspond with the crystalline 

 angles of the mineral orthoclase which constitutes more than half its 

 bulk ; so that if any considerable portion of the crystals are arranged 

 a definite direction in the rock, we might expect the mass 011 

 rinking to divide by joints in planes defined by crystalline struc- 

 This has yet to be proved ; but it is probable that nearly all 

 oints in igneous rocks are due to the combined influence of these 

 uses. 



Direction of Fissures. In examining with attention a consider- 

 ,ble surface of rock, it w r ill be found that amongst the joints are some 

 ore open, regular, and continuous than the others, which occasionally 

 together stop the cross-joints, themselves ranging uninterruptedly 

 or some hundreds of yards, or even for greater distances. There may 

 more than one such set of long joints, and, indeed, this is corn- 

 only the case ; yet, generally, there is one set more commanding than 

 .e others, more regular and determined in its direction, more com- 

 letely dividing the strata from top to bottom, even through very 

 eat thicknesses and through several alternations of rock. For 

 xample, there is a peculiar character of joints in each of the principal 

 strata of the mountain limestone series, limestone, sandstone, shale, 

 and also in the sandstones shales and coal of a coal district ; yet, 

 throughout the whole of Yorkshire, all these rocks are divided by the 

 master-joints passing downward through them all in nearly the same 

 direction, north by west and south by east. These master-joints, called 



