254 N. S. SHALER — FORMATION OF DIKES AND VEINS 



this inquiry the phenomena of dikes will first he considered ; after that, 

 those of veins. 



Dike Fissures 



classes of dikes 



Even a cursory examination of dike phenomena will show the ohserver 

 that so far as the fissures which these structures occupy are concerned 

 they are divisible into three grou[)s : Those which clearly follow the 

 planes of preexisting joints, those which are developed along the bedding 

 planes of stratified rocks, and those which more or less deviously Ijurrow 

 their way through the country rock without availing themselves of either 

 of the kinds of incipient cracks which may exist in the rocks they 

 traverse. 



MODES OP OCCURRENCE 



Not infrequently there may occur in the same field of dike action two of 

 these types of fissures. More rarely all three of them may be found asso- 

 ciated within a small area, sometimes in such close juxtaposition that a 

 single hand specimen may exhibit the several t3q:)es of penetration. It 

 not infrequently occurs that the dike which forces its way upward by 

 developing the joint planes here and there extends laterally through the 

 bedding planes for great distances, and this when the beds are horizontal 

 and the intruding sheets of exceeding tenuity ; they ma}^ indeed, not 

 exceed a few millimeters in thickness. Under certain rarely occurring 

 conditions a dike intruded between layers of sedimentation may, as Gil- 

 bert has shown, cease to move onward as a sheet of even thickness. 

 When this process of development is arrested before the thrust from below 

 ceases to operate, the relief is accomplished by the process of uplifting 

 the overlying masses forming a local vertical enlargement of the intruded 

 lava in the form well termed " laccolite." Such upward movement of 

 the overlying beds most likely occurs in the intrusion of all the greater 

 sills, though some of them of smaller size may be formed without any 

 uprising of the surface, the space being won through the compression of 

 the rocks, due to the in-wedging action of the dike. This is probably 

 the commonest form of relief in the case of the ordinary vertical or joint- 

 following dikes. 



Effect of Heat on Walls 



It is noticeable that in nearl}'^ all dikes save those of exceptional!}'' 

 great width the effect of the heat of tlie material on the walls of countr}"- 

 rock has been but slight. It is rarel}'' sufficient to produce any fusion 



