MINERAL VEINS 265 



(2) Co??ipression Joints are caused when the rocks yield along 

 the shearing planes. In simply folded strata are produced two sets 

 of strike joints which are inclined toward each other, but whether dip 

 joints will be made by complex folding is not certain. In certain 

 conglomerates the joint planes pass through the hard quartz pebbles 

 and leave a smooth, even, shining face. Tension would pull such 

 a pebble out of its socket and only by shearing could it be cleanly 

 cut. Compression joints are merely a special case of fissility. If 

 the division planes be many and close together, they constitute 

 fissility \ if more widely spaced, jointing. 



The whole subject of joints in sedimentary rocks is a difficult 

 one and the explanations given of them are not altogether satisfac- 

 tory, for several other agencies may be involved in their produc- 

 tion. It is, however, highly probable that the master joints which 

 roughly follow the strike and dip of the strata, have been caused 

 by the forces which produce folding. 



Joints cannot occur in the zone of flowage, and are best devel- 

 oped in the zone of fracture, being of less importance in the transi- 

 tion belt between the two. 



Folds and faults, cleavage, fissility, and joints may all be re- 

 garded as the varying products of the same set of forces, lateral 

 compression and gravity. Just what type of structure is to result 

 depends upon the circumstances under which the forces are ap- 

 plied, the nature of the rocks affected, the depths to which they 

 are buried, etc. Joints and fissility are minute, incipient faults, 

 and whether a rock is flexed or faulted is determined by its rigid- 

 ity, the load which it carries, and the gradual or sudden applica- 

 tion of the lateral compression. 



Mineral Veins 



The gaping faults and fissures which traverse hard rocks gener- 

 ally remain more or less open for a time, and are frequently filled 

 up by a subsequent deposition of material, quite different from 

 the rock which forms the walls of the fissure or fault and which is 

 called the " country rock." Fissures thus filled by crystallized 

 deposits are called mineral veins, which may be either simple or 



