258 N. S. SIIALKll — FORMATION OF DJKES AND VEINS 



It must be acknowledged that these ])rocesses, tliough essential to the 

 hypothesis, are somewhat conjectural, but they api^ear to be in the field 

 of legitimate su[)i)osition. 



VARIATION IN MOISTUIiE OF ROCK STRUCTURES 



Reviewing the phenomena of dikes in the light of the above noted 

 working hypothesis, we see that they become more intelligible than the}' 

 now are. Thus, as those who have studied in deep mines have learned, 

 there is a great difference in the extent to which joints and bedding 

 planes contain water. Some are quite dry, others not. It is not unlikely 

 that this di (Terence continues to a much greater depth than our observa- 

 tions extend. The difference is even greater in the case of bedding 

 planes where the water spaces are likely to be more extensive and where 

 the la^^ers of rock may be chai'ged with the fluid. The variations in this 

 feature are obviously great, some sections of strata having the separate 

 members loosely united, while in others they are firmly knit together. 

 Considering these differences, we may conceive that when dike materials 

 seek to force their way upward they may be guided in this movement 

 by the aid which they receive in opening their paths through the action 

 of the ex[)anding vapors. Where the vapors are readil}^ generated in 

 the bedding planes and not in the joints, the lava will be deposited as 

 sills. Where the joints tend to be opened by the steam while there is 

 no like action in the bed planes, no sills will be formed. Where, as is 

 not infrequent!}!' the case, the dike has evidently encountered difficulties 

 in breaking its path upward, being forced to make its wa}^ without much, 

 if an}^, assistance from the joint fissures, either because they are not well 

 develoi)ed or because they are very tight, we should expect to find, as we 

 do in fact often find, that the sills are well developed. In the case of 

 the dikes of the above-mentioned district near Helena, Montana, this 

 feature of burrowing dikes evidently formed with difficulty, from which 

 extend slender sills, which appear to have been developed with relative 

 ease, is very ai)parent. 



Condition of Sills 



So far as my observations extend, the history of diking in every well 

 stratified section shows that when sills are formed they are commonly 

 produced in the first invasions of the igneous rocks, the later injections 

 tending to follow the first planes. This is the order which we should 

 expect if the contact of water with the heated injections was of impor- 

 tance in developing the fissures, for where the layers are so related to one 

 another that there is a storage of water between them, the quantity thus 



