DEFORMATION IN ZONE OF FRACTURE AND FLOW AGE. 1009 



cementation, oxidation is frequently confined to those parts of the belt in 

 which the waters come somewhat directly from the surface; whereas deeper, 

 especially where the percolating wafers have come into contact with organic 

 compounds or sulphides, the waters may become reducing, and partial or 

 complete deoxidation of the valuable metals take place. Furthermore, 

 while solution probably occurs to a greater extent than deposition in the 

 belt of cementation, as explained on pages 629-640, material is continually 

 deposited in the openings of the belt, and the rocks are cemented in conse- 

 quence of the expansion reactions of oxidation, carbonation, and hydration, 

 and in consequence of selective precipitation, of intrusion, and of other 

 phenomena. 



ZONE OF COMBINED FRACTURE AND FLOWAGE. 



Deformation by combined fracture and flowage takes place in a middle 

 zone because the rocks of a given part of the lithosphere vary in character, 

 temperature, moisture, speed of deformation, etc. (See pp. 766-768.) Of 

 these factors producing combined fracture and flowage, the effect of the 

 chnracter of the rocks is most obvious. While very considerable rock 

 masses, such as the limestones and quartzites, may be homogeneous, 

 heterogeneit}^ is the rule. The sedimentary rocks are composed of laminae., 

 layers, beds, and formations, no one of which is exactly like the adjacent 

 one. The massive igneous rocks contain minerals of different resisting 

 power, the particles of which are of different sizes. In consequence there 

 is a broad zone in which the deformation may be by combined fracture 

 and flowage. For instance, in an interbanded slate and graywacke the 

 slate bands may be deformed by flowage and the more rigid g-raywacke 

 bands be deformed largely by fracture. In an interbedded limestone and 

 clastic series the limestones may be deformed mainly by flowage and the 

 clastic material mainly by fracture. In many cases the change from one 

 kind of deformation to the other is amazingly sharp, the more resistant 

 bands being intersected by innumerable fractures which stop abruptly at 

 the bands where the rock is deformed by flowage. The deformation of a 

 massive rock mainly by flowage, but in a subordinate way by fracture, is 

 finely illustrated by the Berlin rhyolite-gneiss described by Weidman. a The 

 deformation of this rock was mainly by recrystallization, but many of the 



"Weidrnan, S., A contribution to the geology of the pre-Cambrian igneous roeks of the Fox Kiver 

 Valley, Wisconsin: Ball. Wis. Geol. Nat. Hist. Survey, No. 3, 1S98, pp. 32-47. 



MON XLVII — 04 64 



