94 



LITHOLOGICAL GEOLOGY. 



the fold overhangs the base, and the pitch of the beds on the right is 

 thereby reversed, so as to be like that on the left. In rare cases, an 

 abrupt fold has been so compressed below that the beds above diverge 

 fan-like. 



Fig. 100 represents a section (by Heim, p. 833) from the Swiss side 



Fig. 100. 



Section east of L. Lucerne, extending south, 15 m., through Windgalle (4, to the right), a peak 

 10,455 feet high ; 1, Gneiss ; 2, Triassic beds ; 3, Lias ; 4, Jurassic, above the Lias ; 5, Cre- 

 taceous ; 6, Eocene Tertiary, including Nummulitic beds. 



of the Central Alps. To the right, the strata, 1, 2, 3, 4, 6, are in an 

 overturn fold, the newest, 6, beneath 4, 3, 2, 1, with 1, the oldest, at top. 

 Through the dotted lines, the forms of the folds in the section may be 

 traced. For other sections, see pp. 212 to 214, 346, 399, 785 to 790. 

 The ridge-line of a fold is usually inclined, and unequally, either way ; 

 and when this inclination is large, the steeper end of the fold is gener- 

 ally wanting because of removal by denudation. See Fig. 108, p. 98. 

 113. In describing 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 



