WORK OF INJECTION. 647 



Injection by occupying the openings of the rocks indurates them. In 

 these respects the process is similar in its results to cementation. The 

 injection magma is a solution which crystallizes. This is a freezing of the 

 solution. (See p. 1 1 3.) In cementation the solids crystallize from the solu- 

 tions. It is believed, as will be seen (pp. 723-728), that the process of 

 injection passes by gradation into cementation. The forms which injec- 

 tions take are often similar to those of the veins produced by cementation, 

 and rock masses formed by either process may have the same relations to 

 the previous structure of the rock. 



Injecting materials comprise magmas of all kinds. Injections follow 

 parallel and irregular fractures alike. Where the injections are in plane 

 parallel openings, such as those of faults, joints, and fissility, the rock may 

 take on a banded character. These bands may be large and wide apart, as 

 is often true of injections along faults; smaller and closer together, as is 

 frequent along joints; or very narrow and very close together, as is occa- 

 sionally the case along planes of fissility. The banded rocks produced by 

 injection may have an added complexity, due to the fact that many of the 

 dikes follow one or more sets of fractures diagonal to the previous structure 

 as well as parallel to it. Where the injections are in rocks with original 

 irregular fractures, such as brecciation, this results in great complexity of 

 structure. 



The Sierra Nevada granite furnishes an admirable example of a rock 

 which has been injected in a complicated maimer. This granite is cut by 

 several sets of intersecting joints. Many of these joints have been taken 

 advantage of by the later injecting granite ; and in the magnificent 

 exposures in the region the dikes may be seen in parallel sets, intersecting 

 other parallel sets of dikes. One of the localities in which the phenomena 

 may be particularly well seen is the Yosemite Valley. Here there are at 

 least six sets of intersecting joints. At some places two or three of these 

 sets of joints have been taken advantage of by the entering material. 



In some regions, after one set of fractures has been taken advantage of 

 by injection material, succeeding orogenic movements have produced other 

 sets of fractures, which have again been injected, and in some regions there 

 may be found evidence of several distinct periods of fracturing and injec- 

 tions Complicated injection is particularly likely to occur adjacent to 

 great laccoliths within the zone of fracture. 



