OPENINGS OF ZONE OF FRACTURE. 1007 



FORM AXD CONTINUITY <IF OPENINGS. 



The openings in rocks include those which are of great length and 

 depth, as compared with their width, and thus are essentially flat p'arallelo- 

 pipeds; those in which the dimensions of the cross sections of the openings 

 are approximately the same, and therefore resemble tubes of various kinds; 

 and those which are irregular. 



The openings which have great length and depth as compared with 

 their width are those of faults, of joints, of bedding partings, and of fissility. 

 The order mentioned is that of continuity. As to position, bedding partings 

 are parallel to previous structures; while faults, joints, and fissility are at 

 various angles with the bedding parting, and therefore intercept the layers. 

 Because of this fact, these forms of openings may connect separated porous 

 strata. This is more likely to be true of faults than of joints, and of joints 

 than of fissility. 



The openings in which the dimensions of the cross section are approxi- 

 mately the same are those in mechanical deposits, such as conglomerates, 

 tuffs, sandstones, and shales. From the point of view of ore deposits the 

 most important characteristic of the openings of this class is that they are 

 continuous, and therefore any part of a coarse, uncemented mechanical 

 deposit is connected with all other parts by openings. 



Irregular openings are those of the vesicular lavas, and the irregular 

 fractures of_rocks The openings of the lavas are usually variable in mag- 

 nitude, and are discontinuous. Usually single irregular fracttfres are of 

 limited extent, but many fractures may be concentrated along a zone, and 

 a composite zone may be continuous for long distances. 



Openings of any of the above classes, whether produced by deforma- 

 tion, by original sedimentation, or by volcanic action, may be enlarged by 

 solution. This will be the case wherever the processes of solution more than 

 counterbalance the processes of deposition, and, as later explained, is more 

 likely to occur with downward-moving water than with upward-moving 

 water. Since downward-moving waters are dominant above the level of 

 ground water, and are prominent in the upper part of the belt of cementation, 

 it is in this area that openings are most frequently enlarged by solution. 



It has been argued by Posepny that openings serving as channels for 

 ground water may be produced wholly by solution. That openings may 

 be somewhat prolonged and adjacent opening connected by solution, 



