96 



LITHOLOGICAL GEOLOGY. 



When only the under surface of projecting strata can be reached, the upper side of 

 the instrument {a b, in Fig. 102) should be applied to the rocks. By holding the in- 

 strument between the eye and the sloping outline of a distant hill or mountain, mak- 

 ing a b or c d coincide with this outline, the angle of slope may be measured. The 

 s'rike of inclined strata, when the\ r are seen in profile, may be taken by holding the 

 instrument with the edge a b horizontal (as ascertained by the pendulum), and then 

 sighting along a b and finding thus a point on the edge of the sloping layers (or in the 

 line of such an edge produced downward, if the rocks are above the level of the eye); 

 the direction of this point is the strike. Then, by making the edge a b to coincide, by 

 sighting across, with the slope of the layers, the dip may be taken. Before applying a 

 clinometer to a layer of rock, a strip of board should be placed upon the layer, so that 

 any unevenness of the surface may not lead to error. 



The directions obtained by a compass will always need correction for the magnetic 

 variation. 



Faults. — The term fault is defined on p. 93. In Fig. 9 6, the parts 

 of each faulted bed are of equal thickness on the two sides of the line 

 of fault. When, in a dislocation, there is a lateral or oblique shove, as 

 is often the case, the thickness may differ, provided the bed is not 

 throughout of uniform thickness. This difference of thickness may in- 

 dicate a lateral movement when there is no proof of a vertical. 



Complexities in stratified deposits arising from denudation and other 

 agencies. — By the denuding action of waters, strata are removed over 



Fig. 103. 



Fig. 104. 



^^ 



extensive territories, the tops or sides of folds are carried away, and 

 various kinds of sections made of the stratified beds, which are often 

 perplexing to the student. 



One of the simplest of these effects is the entire removal of the 

 rocks over wide intervals, so that the continuation of a stratum is met 

 with many miles distant, as in Figs. 103, 104 ^ 



The result is more troublesome among the flexed or folded strata. 

 A series of close flexures, like Fig. 105, worn off at top down to the 

 Fig. 105. Fig. 106 a. Fig. 106 b. 



1 2 3 3'2'1 



2 3 3'2' U 



line a b, loses all appearance of folds, and seems like a series of layers 

 dipping in a common direction. This is best seen from a single fold 



