94 



LITHOLOGICAL GEOLOGY. 



may rise beyond in the same line, or else in another line to one or the 

 other side, making often overlapping series. Thus all the positions 

 represented in the lines in Figs. 11 to 16, p. 19, may occur; and, in 

 fact, they, for the most part, do occur in the Appalachian range. 



The two slopes of a fold may be alike ; or, as in b, c, d, e, one 

 may be much steeper than the other. The line a x shows the posi- 

 tion of the axial plane of the fold in each case. 



The ridge-line of a fold may be horizontal, but more commonly it is 

 inclined, said reaches gradually its greatest elevation. 



Such are some of the various conditions which have been observed, 



Fig. 100 



especially in mountainous regions. Fig. 100 represents a section, by 

 Logan, from the Archaean rocks of Canada. The folded rocks are 

 often overlaid by others of more recent date. 



113. In describing the positions of strata, the following terms are 

 used : — 



a. Outcrop. — A ledge or mass of rock coming to the surface, or 

 cropping out to view at the surface or above it (Fig. 101). Outcrop- 

 ping edges are sometimes called basset-edges. 



Fig. 101. 



b. Dip. — The slope or pitch of the strata, or the angle which the 

 layers make with the plane of the horizon ; as ap (Fig. 101). The 

 direction of the dip is the point of the compass toward which the strata 

 slope ; for example, the dip may be 25° to the southeast, or 15° to the 

 west, and so on. 



c. Strike. — The direction at right angles with the dip, or the course 

 of a horizontal line on the surface of the inclined beds, as s t. 



The strike and dip are always observed with care, in the study of strata; for the 

 strike is in general at right angles approximately to the direction of the force that up- 

 turned the beds, and indicates therefore an important fact with regard to the origin of 

 the upturning; and the dip is but little less important, since it illustrates the amount 



