CAUSES OF DIVERSITY IN DIKES 257 



In the greatest number of instances the opening of the space occupied 

 by the sill must have been accomplished by the bodily uplifting of the 

 overlying mass of strata ; yet the inducement of lessened pressure could 

 not apparently have been the onl}^ action which led to the formation of 

 the far-extending and often relatively very thin sheet of dike matter. 



In some examples the sills may be observed to deflect somewhat 

 downward from their original pathway, when the extended overlying 

 beds should have afforded an easier passage by way of the joints. 



EFFECT OF WA TEE 



Unable to find any other working h3^pothesis concerning the develop- 

 ment of the paths of dike injections which aids us in understanding the 

 curious conditions of their movements, I have ventured on the conjec- 

 ture that their course is in part at least determined by the presence or 

 absence of water in the incipient crevices — ^that is, the joints or bedding 

 planes — or in the relative quantity of that material in those planes. Let 

 us suppose that the molten rock comes in contact with such an incipient 

 fissure containing vv^ater, either free or in connection with the rock on 

 either side of the crevice. The effect would be to vaporize this water, or, 

 if the pressure at the given depth were too great to permit vaporization, 

 at least to bring about an expansive strain, which would help to open the 

 space to the entrance of the lava. Should the pressure engendered by 

 this process of vaporizing be less than that which impelled the igneous 

 matter from below, as would doubtless usually be the case, this matter 

 would begin to enter the rent. If the pressure due to the vapor were by 

 chance greater than that impelling the lava it would for the time be de- 

 barred from that path. It is perhaps not to be supposed that in ordinary 

 conditions the pressure due to the expansion of water vapor, even in the 

 high temperature which the presence of the fluid rock would occasion, 

 would of itself bring about the opening of the crevice, but this action 

 might cooperate with that of the diking material in determining the 

 course it would follow. 



At first sight there appears to be a fundamental difficulty in this hy- 

 pothesis, in that it does not provide a way by which the vapor of the 

 water generated in the fissure could be disposed of. On consideration, 

 however, it seems eminently probable that relief might be found in either 

 or both of two ways. The stratified rocks would be, it may be assumed, 

 more permeable to vapor than they are to the injected fluid, so that there 

 might be some, though most likely a slow, escape provided in that man- 

 ner; it is even more likely that the vapor would be absorbed into the 

 molten rock either mechanically or in connection with chemical changes, 

 or these actions, resulting in the removal of the vapor, may be combined. 



