210 Van Hise and lloskins — Pre-Cambrian Geology. 



The foregoing principles are applied to the Appalachian and 

 certain other regions. Much of the Appalachian region is one 

 of monoclinal folding, and, therefore, one of monoclinal cleav- 

 age, and of monoclinal iissility secondary to cleavage. In areas 

 in which there have been secondary impregnations and injec- 

 tions there is a secondary monoclinal banding. A similar 

 explanation is applied to the monoclinal banding of the Archean 

 gneiss in great areas in Canada and in southwestern Montana. 



Joints. — Joints may be classified into tension joints and com- 

 pression joints. The first ordinarily develop in planes normal 

 to the tensile forces, the second develop in the shearing planes 

 inclined to the compressive forces. Tension joints are often 

 produced along the crowns of the anticlines in areas of simple 

 folds. In areas of complex folds tension joints may be pro- 

 duced in two directions at right angles to each other. Com- 

 pression joints, as explained by Daubree and Becker, form 

 along shearing planes, and are in the simplest case produced 

 simultaneously in two directions nearly at right angles to each 

 other. Joints, implying as they do openings in the rocks, are 

 necessarily confined to the outer zone of fracture and the middle 

 zone of fracture and flowage. 



Faults. — Faults may be classified into normal or gravity 

 faults and reverse or thrust faults. The first class is ordinarily 

 produced under conditions of tension, the second under condi- 

 tions of compression. J^ormal or gravity faults result in the 

 dilation, thrust faults result in the contraction of the part of 

 the crust of the earth affected by them. It is shown that the 

 average deformation of a region may be the same whether it 

 be by a few great faults with little or no fissility, by more fre- 

 quent lesser faults with or without fissility, by faults and over- 

 folds with or without both cleavage and fissility, or by folding 

 with or without faults and cleavage ; also that there is every 

 gradation between faulting and fissility, and probably every 

 gradation between faulting and cleavage. Faults are limited 

 in horizontal as well as in vertical extent, and are usually con- 

 fined to the zones of fracture and of fracture and fiowage. 

 Probably most faults at sufficient depth pass into flexures, and 

 deeper down these flexures may die out. 



Folds, cleavage, fissility, joints, and faults are regarded as 

 the conjoint products of thrust and gravity. Similar forces 

 acting upon heterogeneous rocks under various conditions pro- 

 duce diverse phenomena. Thus several classes of phenomena 

 which are often treated as independent and unconnected are 

 genetically connected. In the zone of combined fiowage and 

 fracture all the structures occur together in a complex manner, 

 the particular combination of phenomena depending upon the 

 relative thickness, strength, and brittleness of the rock beds 

 concerned, and upon the forces at work. 



