256 N. S. SilALER FORMATION OF DIKES AND VEINS 



In these cases, although the bedding- i)lanes may l>e extremely well de- 

 veloped, so that the layers may be separated with the greatest ease, while 

 the joints are very closely ad pressed, the intrusions persistently follow 

 the paths of these close fissures, entirely neglecting the opportunity to 

 extend between the beds. 



Combination Dikes 



In yet other and exceptional occurrences a dike may be formed which 

 has burrowed its way upward without the advantage afforded it by 

 following distinct joint planes, while from point to point it has given off 

 sills which have entered readily between the strata, extending it may 

 be into very thin sheets. Excellent examples of such combinations of 

 dikes, in which great opportunity of vertical extension through layers of 

 rock is combined with exceeding ease in the development of sills, may 

 be observed in the highly metamorphosed slates along the Missouri 

 river, near Helena, Montana, especiall}^ in the boulders at the almndoned 

 placers on the banks of that stream. 



Causes of Diversity in Dikes 

 effect of rock structure 



The wide differences in the behavior of injected lavas when they cover 

 stratified deposits appear to indicate some conditions serving to deter- 

 mine their path other than those afforded by the force which forces them 

 up.ward or the extent to which the walls of the joint or bedding planes 

 adhere. The facts seem to indicate that in certain cases these joint fis- 

 sures readily open and give passage to the fluid rock, while in others, in 

 no evident manner different, they resist the entrance of the dike matter 

 in an obstinate way. The question is as to the cause or causes of these 

 variations. 



The first suggestion that may be made as to the cause of the diversity 

 whicli we note in the paths of dikes through stratified rock is that it is 

 due to the relative ease with which the fluid rock forced its way toward 

 the zone of diminished pressure at or near the surface. This view is, 

 however, not reconcilable with observations which may be made iii any 

 region where sills abound. It ma}^ be there seen how improbable it is 

 that the igneous matter gained in nearness to the surface by extending 

 into the sills. 



In certain cases where the beds are tilted the extension along the up- 

 ward inclined bedding planes may have furnished a way toward the air ; 

 but such conditions are rare. 



