558 



KIRK BRYAN 



bedding planes, faults, columnar joints, and openings formed on 

 slaty cleavage, fissility, and schistosity are the common types of 

 fractures in igneous and metamorphic rocks. In sedimentary 

 rocks joints, bedding planes, cross-bedding planes, and faults are 

 the principal types of fractures. If the fractures are very numerous 

 and closely spaced, however, water moves through the rock in 

 essentially the same fashion as in porous rocks. Minute fracture 

 systems add very much to the effectiveness of interstitial pores and 

 are probably present in all porous rocks. The essential difference 

 is that in a porous rock water moves bodily, and, as a rule, slowly 



Fig. 23. — Diagram showing occurrence of springs near Mount Carmel, Con- 

 necticut. (After Gregory.) 



through minute openings, whereas in fractured impervious rocks 

 it moves more rapidly as sheets, threads, or films of water. 



Fracture springs may be divided into two classes according to 

 the attitude of the fractures toward one another and toward the 

 horizon. A common joint system is one in which there are two or 

 three sets of joints at right angles to one another and one of the 

 sets is horizontal. Springs due to such joints may be called 

 quadrille fracture springs (Fig. 22 a). Such a jointing system is 

 common in sedimentary rocks and in impervious igneous rocks, 

 particularly in sheeted plutonic rocks. Springs due to two systems 

 of joints at right angles to each other and inclined to the horizon 

 may be called Crosshatch fracture springs (Fig. 22 h). Joints and 

 bedding planes in Triassic sandstones- form such a system of 



