Handbook of Paleontology 53 



zontal position a recumbent anticline is developed. Fan- 

 shaped folds are formed when in anticlines the lower part 

 of the limbs are pinched together or compressed and the 

 upper part bulges beyond on both sides. An anticline 

 with short axes is known as a dome fold or dome. The 

 longitudinal and transverse axes may be about equal or 

 the longitudinal axis may be many times longer. There 

 are similar varieties of synclines. A syncline correspond- 

 ing to a dome is known as a basin fold or structural basin. 

 Isoclinal anticlines and synclines are formed when there 

 is close regular folding in which the limbs of all the folds 

 have practically the same inclination. It can be readily 

 understood that erosion in folded regions will bring to 

 light very complicated structures. There will be repeti- 

 tion .of beds, reversal of beds etc.; the more complicated 

 the folding, the more complicated the surface structures 

 will be. Folds may be simple or complex. A mountain 

 region may be composed of a series of anticlines which 

 together form a large arch. Such a compound anticlinal 

 series is known as an anticlinorium. A similar compound 

 synclinal series is known as a synclinorium. 



The earth's crust is traversed in all directions by frac- 

 tures, and they are found in all classes of rocks. These 

 fractures are known as rifts or fissures when they are 

 large and traverse many adjoining rock masses for great 

 distances. When they are smaller in size and confined 

 to a single rock mass or set of strata they are known as 

 joints. Joints are present in the rocks in systems, that is, 

 they run as divisional planes or surfaces in definite direc- 

 tions more or less parallel one to the other. There may 

 be two directions of jointing, or three or even more, and 

 they divide the strata into closely fitting blocks. Finer- 

 grained rocks such as shales and limestones show more 

 perfect jointing. 



