200 N. S. SIIALKIi FORMATION OF DIKES AND VEINS 



widely 0})en liHsuras; such rents have occasional!}'' l)een ol)serve(l in 

 mines at considerable depth beneath the surface, but within the zone 

 where the j^reater part of tlie vein ])uilding takes place we may fairly 

 presume that the pressure is such as would prevent the develo|)ment of 

 freely open crevices. Such, indeed, could not be maintained at the depth 

 of even 3 miles below the surface, except they were filled with water which 

 had no chance to escape upwardly as fluid or vapor. The only kind of 

 fissure whicli could, except for the presence of water, be maintained at 

 any great depth would be such as were formed by faults, the walls of 

 which were not perfectly parallel — a condition of all faults — so that when 

 slipped apart one from another, though closely adpressed, a winding 

 cavity would be formed by the contact of the warped surfiices ; but it is 

 doubtful if even such openings could be maintained at the depths where 

 most veins are deposited. The well known fact that veins usually con.- 

 tain here and there masses of the country rock which have been forced 

 out of position laterally by the formation of the vein matter, but have 

 not fallen downward, and that in no case within my observation has a 

 mass of the country rock disai)[)eared in a way to suggest such down- 

 falling, shows pretty clearly that the deposition in wide veins rarely if 

 ever occurs in previously open fissures; that is, those in which there is 

 a cavity having an area anything like that occupied by the completed 

 vein.- It therefore becomes a question as to the mode in which the vein 

 material finds its way to the place of deposition. 



ILLUSTRATION OF VEIN-FORMING FROM GEODES 



Some light on the foregoing question appears to be afforded by ob- 

 servations which may be readily made on the formation of geodes. As 

 is well known, these bodies in their t3''pical form are spheroidal masse?, 

 usually of quartz, which are formed essentially in the manner of veins. 

 They mav, indeed, be termed globular deposits in this class ; in fact, 

 by extending the inquiry over a large field I have been able to trace a 

 tolerably complete series of forms from spherical geodes to ordinary fis- 

 sure veins, a series sufficiently without breaks to warrant the assumption 

 that all these bodies belong in one category. A study of these geodes as 

 they occur in Kentucky and elsewhere, especiall}'' in the shales of the 

 sub-C!arboniferous rock, has afforded me some interesting and instruct- 

 ive suggestions concerning the i)rocess of vein-making which I will now 

 briefly set forth. 



Normal geodes are hollow spheroids and are generally found in shales. 

 They clearly represent in most cases a segregation of silica, which has 

 evidently taken place under conditions of no very great heat, brought 



