ARCHITECTURE OF FRACTURED SUPERSTRUCTURE 65 



of them where the facts are more nearly adequate may cor- 

 respond. In Fig. 48 is represented the areal geological map con- 

 structed from the field map, with the doubtful area at one side left 

 blank. 



Some conclusions from this map may now be profitably con- 

 sidered. The complexly folded sandstone formation at the left 

 of the map appears as the oldest member represented, since its 

 area has been cut through by the intrusive granite which does not 

 intrude other formations, and is unconformably overlaid by the 

 limestone and its basal layer of conglomerate. The limestone in 

 turn is unconformably overlaid by the merely tilted sandstone 

 beds at the right of the map. These three sedimentary forma- 

 tions clearly represent decreasing amounts of close folding, from 

 which it is clear that each earlier formation has passed through 

 an episode not shared by that of next younger age. Of the other 

 intrusive rocks, the dike of porphyry is younger than all the other 

 formations, with the possible exception of the upper sandstone. 

 Offsetting of the formations has disclosed the course of a fault, 

 and from its relations to the dikes we may learn that of these the 

 porphyry is younger and the basalt older than the date of the 

 faulting. 



The dashed lines upon the map (AB and CD) have been selected 

 as appropriate lines along which to construct geological sections 

 (Fig. 48, below map), and from these sections the exposed thick- 

 nesses of the different formations may be calculated. In one in- 

 stance only, that of the conglomerate, can we be sure that this 

 exposed thickness measures the entire formation. 



Fold versus fault topography. The more resistant or " stronger " 

 rock beds, as regards attacks of the atmosphere, in the course 

 of time come to stand in relief, separated by depressions which 

 overlie the " weaker " formations. Simple open folds which are 

 not plunging exercise an influence upon topography by producing 

 generally long and straight ridges. More complex flexures, since 

 they generally plunge, make themselves apparent by features 

 which in the map are represented by curves. Fracture structures, 

 and especially block displacements, are differentiated from these 

 curving features by the dominance of straight or nearly rectilinear 

 lines upon the map. The effect of erosion is to reduce the asperity 

 of features and to mold them with flowing curves. The frac- 



