Handbook of Paleontology 51 



with the ends of the semicircle on a line parallel to the 

 upper and lower edges. At the center of this line fasten 

 a thread by a pin or in some other way and attach at the 

 other end a small weight so that it hangs below the arc 

 but above the edge of the board or cardboard. The 

 thread then should cut the graduated semicircle exactly 

 in the middle, which is zero. Each end of the semicircle 

 is marked 90° and the arc on each side of the zero point 

 is then marked off into the correct number of spaces, ten 

 degree spaces first, then five, then one. If preferred one 

 space may be used for even- two degrees. A needle may 

 be attached to the thread instead of a weight and used 

 as a pointer. The width of the outcrop or exposure of 

 rock at the surface depends upon the angle at which the 

 strata are dipping. When they are vertical the edges of 

 the strata are exposed and the width of the outcrop will 

 be the thickness of the beds. The more nearly horizontal 

 the strata are the greater the width of the outcrop on a 

 horizontal surface. A trained geologist can calculate the 

 thickness of a bed of rock when he has the angle of dip 

 and the width of the outcrop. 



There are three principal kinds of folds : anticlines., 

 synclines and monoclines (figure 8). An anticline is an 

 arched fold or upfold, opening downward. The sides of 

 the folds are the limbs and the crest line the axis of fold- 

 ing. A syncline is a trough fold or downward fold, open- 

 ing upward. Sometimes in nearly horizontal or gently 

 dipping strata the dip changes from gentle to steep and 

 back again, giving a steplike bend in the strata. This is 

 known as a monocline. The axis of an anticline or syn- 

 cline may extend in a horizontal position for some dis- 

 tance and then dip into the ground and die away. This 

 dip of the anticline or syncline is known as the pitch, and 



