W. 0. Crosby — Absence of Joint-Structure at Great De^Jths. 417 



undoubtedly a correct expression of the prevailing opinion among 

 geologists concerning the lower limit of the joint-structure. A 

 joint, of course, is always a plane of division, not a theoretical or 

 potential plane, as in crystalline cleavage, but an actual physical 

 break, of which the quarry man and miner consciously or uncon- 

 sciously take advantage. And the joint must be almost equally a 

 source of weakness in the rock, whether it exists as an open crack 

 near the surface, or as a merely mathematical plane at greater 

 depths. Now it seems probable that the miner judges of the 

 persistence or non-persistence of the joint-structure in depth, not 

 so much by the presence or absence of visible planes of division, as 

 by the ease with which the rock is broken and removed. If this is 

 so, then it is clear that the criticism embodied in the foregoing 

 quotation from a standard authority fails to cover the point; and 

 the relation of jointing to depth must be regarded as an open 

 question still. 



Geologists are generally agreed that the two principal causes of 

 joint-structure are contraction of the rocks and movement of the 

 rocks ; and the contraction, speaking generally, must be ascribed 

 either to the consolidation of sedimentary rocks, the cooling of 

 eruptive rocks, or the crystallization of rocks of either class. 



The notion generally held by geologists that the jointing continues 

 downward indefinitely, perhaps as far as the rocks are solid, may be 

 sound enough when only the joints due to movements of the rocks 

 are considered, but when we take into account the more general and 

 efficient cause of jointing — contraction — this view seems to rest on no 

 better foundation than the supposed fact that no scientific observa- 

 tion or theory points to the contrary conclusion. My present 

 purpose, however, is to show that, even if we reject, as untrustworthy 

 in this instance, the testimony of the miners, there is still good 

 ground for believing that, in the stratified rocks at least, the joint- 

 structure due to contraction is to a very large extent a superficial 

 phenomenon. 



Taking a general view of the deposition of the stratified rocks, let 

 us trace the history of a single stratum, and observe where and when 

 the joint-structure arising from contraction is probably first developed 

 in it. As the process of deposition goes on over the area, this 

 stratum is gradually compacted by the increasing pressure of newer 

 deposits, and we get contraction in the vertical direction ; but I 

 think too much stress cannot be laid upon the fact that a vertical 

 pressure, acting directly, must be entirely inoperative so far as the 

 development of joint-structure is concerned, since this demands lateral 

 contraction. However, as the pressure imposed upon the stratum by 

 later sediments increases, so does the temperature, through the rising 

 of the isogeotherms ; and this higher temperature, co-operating with 

 the pressure, effects the expulsion of a portion of the water with 

 which the material was originally saturated. The loss of water from 

 the stratum means, of course, a tendency to contract in all directions ; 

 though still chiefly in the vertical direction, owing to the controlling 

 influence of gravity. Thus it appears that only a modicum of the 



DECADE 11. — VOL. Till. — XO. IX. 27 



