34 C. R. VAN HISE 



which the parallel injections are also found. Scores of illustra- 

 tions of irregular intrusions, so extensive as to occupy important 

 or even major parts of various districts, could be mentioned of 

 any of the great groups of rocks. A number of instances are 

 given on pages 49-50. 



All openings which may be taken advantage of by injection 

 may also be taken advantage of by underground water deposits, 

 and thus by a combination of fracturing and cementation the 

 area of the rocks is increased. That the rocks may thus receive 

 an important extension of their surface area has been noted by 

 Shaler. 1 While parallel and irregular cementation by water solu- 

 tion may not be so important as igneous injection in the lateral 

 extension of rocks, it is a more widespread phenomenon, and 

 undoubtedly has an important effect. Wherever openings have 

 been produced in relatively deep-seated rocks (that is, in the 

 lower part of the zone of fracture, and in the zone of combined 

 fracture and flowage 2 ) it appears to be the rule that cementation 

 follows them, and thus rock material again occupies the entire 

 space. 



In regions where fissility has been developed, the laminae 

 are cemented by layers of infiltrated material, which in many 

 places average as wide as the laminae cemented. This is seen 

 at many localities in the southern Appalachians. Cementation 

 is not more important in closing the spaces between laminae than 

 it is in closing joints, faults, and irregular fractures. Such 

 cementation may be found in the same districts as the deposi- 

 tions along the planes of fissility, and thus double the effect, or 

 it may occur in districts in which fissility is unimportant. The 

 Marquette district of Michigan finely illustrates the latter. 

 Entire formations have been broken by innumerable joints, 

 irregular cracks, or even brecciated. The openings are now 

 entirely closed by cementation. 3 Since the time of this cementa- 



I The crinetic hypothesis and mountain building, by N. S. Shaler : Science, 

 Vol. XI, 1888, pp. 280-281. 



2 Principles of North American pre-Cambrian geology, by C. R. Van Hise : 

 16th Ann. Rept. U. S. Geol. Survey, Pt. I, 1896, pp. 589-594, 601-603. 



3 The Marquette iron-bearing series of Michigan, by C. R. Van Hise and W. S. 



