CONDITIONS AFFECTING FORM OF DIKES 255 



of that rock, even where the constitution of the material is such as to 

 favor melting at anything like the temperature which must have existed 

 in the mass of the lava. It is otherwise where the dike has burrowed 

 its way tlirough the deposits it has penetrated without following distinct 

 joints or bed planes. In these instances there is, so far as I have been 

 able to observe, always abundant evidence that the igneous matter has 

 absorbed or fused a large part of the country rock which occupied the 

 position of the intruded mass. This difference in the effect of the dike 

 materials in the rifting and the burrowing dikes — -that is, those which 

 follow the planes of distinct beds or joints and those which do not do 

 so — is, as we shall see, a matter of much importance. With these fea- 

 tures of dike fissures in mind, let us proceed to consider the conditions 

 under which these fissures are riven. 



Influence of Stratification 



It is commonly though tacitly assumed that there is some distinct 

 injecting action which drives the liquid rock into the fissures of the solid 

 overlying strata, but I am not aware that any eff"ort has been made to 

 show exactly how such an action is effected. Let us imagine a fluid 

 mass, however developed, lying beneath the deep covering of overlying 

 solid material. Are we to conceive that the dikes are formed as the re- 

 sult of a disruptive action, such as that which rends a bursting shell or 

 steam boiler? Clearly such is not the fact, for in that work the whole 

 of the covering would be rent and the escaping lava would attain the 

 surface. This point is made the clearer by the fact that in the formation 

 of laccolites the overlying strata, which by their extension should give pas- 

 sage for the lava of the congested sill, are not often the seat of numerous 

 upward going dikes leading from the main mass. Even when these over- 

 lying rocks are apparently normally jointed, there appears to be some 

 reason why these incipient fractures are not readily opened to the molten 

 rock. It seems, indeed, that in certain cases a section of stratified rock 

 may present conditions which favor the formation of sills extending for 

 great distances in approximately horizontal directions, in directions in 

 which no diminution of pressure is obtained, while a vertical course of 

 the injected materials is to a great extent hindered by the conditions 

 mentioned. 



Influencio of Jointing 



In other instances, the greater number of those in which dikes pene- 

 trate stratified rocks, the path of the dike materials is along the joints, a 

 preference being given to those which lie nearest the vertical position. 



