CONDITION OP SILLS 259 



contained is usually much larger than is held in joints ; whereas it is 

 usually the case in dikes which follow joint planes that we find one plane 

 or set of planes evidently affording a freer way than others, and the 

 selection of the path may be explained by the fact that the amount of 

 water in different sets of joints varied greatly. 



It is to be noted in this connection that in the greater number of dikes 

 the effect of the heated matter on the country rock is, as often remarked, 

 surprisingly small considering the temperatures at which we must sup- 

 pose the diking matter entered the fissure. If, however, we assume that 

 the fluid rock conies in contact with considerable amounts of water the 

 effect would be to lower its temperature. It is true that so far as this 

 vaporized water afterward combined with the molten rock this effect 

 would be diminished ; yet the evidence in general goes to show a lessen- 

 ing of the action of heat, which can best be explained by the supposition 

 that it is taken up in part by the vaporizing process. On the other hand, 

 those dikes of relatively rare occurrence, which have made their way 

 through rocks without following definite rifts, and therefore may be pre- 

 sumed not to have come in contact with other than crevice water, often — 

 indeed we may say quite generally — melt or absorb their walls, as is 

 shown by the absence of parallelism of the walls and also by other 

 evidence. 



It can not be assumed that the considerations which have been ad- 

 duced in the foregoing pages effectively verify the hypothesis above set 

 forth, but they point to the conclusion that further study of the matter 

 may bring about this result. 



Vein Fissures 

 dissimilarity between dikes and veins 



Although the early geologists were inclined to regard dikes and veins 

 as species of one genus, it is evident that they have no other common 

 feature than those dependent on the fact that they alike occupy spaces 

 which the}' have won in rocks in which they did not originally belong. 



UNTENABILITY OP THEORY OF OPEN FISSURES 



As regards the formation of the fissures in which veins are contained — 

 the point with which we are here concerned — the original view was to 

 the effect that these rents were freely open at the time when the de- 

 posits were laid on their walls. This view has been generally aban- 

 doned — at least, in its unqualified form — though it is perpetuated in 

 many of our text books. The evidence concerning the formation of vein 

 fissures goes to show that only rarely could veins have been deposited in 



