COMPLEXITY OF CEMENTATION. 621 



cases from a relatively unimportant amount to predominance. The 

 cementing minerals may permeate the rocks through and through, and 

 fill all the interspaces, microscopic and macroscopic. The result is in 

 many instances to give a most extraordinarily complex structure, the places 

 between the original minerals being filled, the parallel fractures being 

 emphasized by the parallel impregnation, and the whole being intersected 

 by larger masses of holocrystalline interlocking mineral materials, some of 

 which are parallel to the original structures, some of which are diagonal to 

 them, and some of which are parallel for a certain distance and then cut 

 across them. The material filling the larger crevices often has distinct vein- 

 like forms. (PI. II, A.) When examined carefully the material deposited 

 parallel to laminated rocks may be found to follow the folia very closely, or 

 it may be found to follow along them for some distance, then break across 

 one or more, and then follow them again. In proportion as the cementing 

 material follows the lamina? the cement bands are likely to be of approxi- 

 mately the same width. In proportion as there is a tendency for the material 

 to break across the folia the veins usually are of unequal size. 



The cementing minerals filling the openings of fissile rocks are usually 

 different or in different proportions from the minerals of the adjacent lam- 

 inae, and hence there is a tendency to preserve and emphasize the laminated 

 arrangement. Where sets of parallel joints are filled, for a similar reason 

 a parallel structure is produced. Where there are intersecting- sets of par- 

 allel joints the parallel structures of cementation may form simultaneously 

 in two or three directions. It therefore follows that where the openings 

 are of the sheeted parallel kind, in general the process of cementation results 

 in a regular alternation of parallel layers of different kinds, the structures 

 of which have no definite relations to the original bedding. 



lEMEXTIMi SCBSTAXCES. 



The more important cements of rocks may be divided into oxides, 

 carbonates, silicates, and sulphides. 



The most important oxides are silica, iron oxide, and aluminum oxide. 

 The more important carbonates are calcite, dolomite, and siderite. The 

 silicates include both the hydrous and anhydrous silicates, but the former 

 are by far the more important. The only sulphides which are geologically 



