ORIGIN AND DEVELOPMENT OF ORE DEPOSITS. 119 



set of cracks, they will be more likely to occur at right angles to 

 the first, or nearly so, than in other directions, that is, in 

 directions as different as possible from those of the first series, 

 these different strains not having been at all relieved so far. 

 Here then are three sets of main divisional planes, dividing the 

 mass under favourable circumstances into approximately rec- 

 tangular portions. The subsequent cooling and shrinkage of 

 these will give rise to secondary fissures, which are likely to be 

 much less regular and of much smaller extension in most cases. 

 Their directions and positions will be especially modified or 

 determined by such cleavage planes as have been produced 

 in the meantime, or by the definitely oriented cleavage planes of 

 constituent minerals, when these are present. 



If the jointing is due to the causes suggested above, it is 

 evident that the most favourable conditions for their production 

 with regularity will be homogeneity of the rock-substances, 

 comparative horizontality of the masses to be acted upon, and 

 continuity. The first condition depends entirely upon the mode 

 of origin of the rocks ; the second is, with few and unimportant 

 exceptions, at one period, at least, common to all stratified and 

 many eruptive rocks ; while the third is common to all rocks, 

 without exception, until the dividing and disturbing causes have 

 commenced to act. 



Evidently, therefore, the first elevation of a rock-mass would 

 give rise to its more important and best developed main joints. 

 Any subsequent elevation might indeed produce joints, but their 

 directions could not fail to be modified more or less by the effects 

 of the first upheaval. This fact is the basis of the scheme of 

 "Mountain Systems" of Elie de Beaumont, which has been 

 applied to mineral veins by his pupil and successor Leon 

 Moissenet. In all cases we find, as was by them first pointed 

 out, that one of the main systems of approximately vertical 

 fissures coincides in direction with some well-developed system 

 of upheaval in the region under consideration. This may be 

 called the longitudinal system, and it is generally crossed in a 

 direction nearly or quite at right angles by the second or transverse 

 system, the two systems being approximately, if not identically, 

 of the same age. 



