II 4 Ol'TI.IM 5 OF 1 II L1>-GEOL<> 



are quick to seize upon and delineate the hara 



:ies of form which geological boundaries assr. 

 the surface of a < ountry changes from plain to hill, and 

 as the rocks themselves alter in thickness and posr 



Numerous illustrations of the applicability of this 

 principle will occur to every observer in the field. : 

 example, he stands at the higher margin of a rocky valley, 

 along the sides of which inclined beds of sandstone, 

 limestone, or other stratified rocks are exposed, dipping 

 gently down the valley, he observes that the outcrop of 

 each bed does not go straight across from the top of the 

 declivity to the corresponding outcrop on the opposite side. 

 On the contrary, it descends the slope in a slant until it 

 reaches the bottom of the valley, when it turn 

 mounts the opposite slope, thus forming a V-shaped in- 

 dentation on the general line of strike (as in the v. 

 on the south side of the map, Figs. 27 and 28). Now 

 the manner in which these windings of the outcrop of 

 inclined strata and their relation to the form of the 

 ground are expressed upon the geological map is a good 

 test of the skill and delicacy which I have insisted upon 

 as so desirable in the map- work of a field -geologist. 

 Many observers are content to draw the lines of out- 

 crop as straight bars across the valleys, thus making them 

 coincident with the strike. On maps of a small scale, 

 indeed, as above remarked, nothing else is possible. Hut 

 where the scale admits of it, much advantage will be 

 gained by faithfully depicting the curving outcroj. The 

 map then tells its story at once, and brings the relation 

 between geological structure and external form almost as 

 vividly before the eye as a well-made model could do. 



