GEOLOGICAL MAPS 



161 



of successive beds will be in parallel bands, all running in the 

 same direction as the strike. But if the surface of the ground 

 is more irregular than this, the outcrop will become irregular too 

 (Fig. 50). The best way to illustrate these two points is to form 

 a rough model of hills and valleys in clay, and then to plunge a 

 bit of slate or tin plate into it in various positions and at various 

 angles. On withdrawing the plate its intersection with the ground- 

 line of the model may be studied. On the whole, this line of 

 outcrop will be found to increase in irregularity as the bed 

 approaches the horizontal, and as the relief of the clay surface 

 becomes more varied. 



Just as from outside we only see the ends and edges of the 

 bricks and timber of which a house is constructed, so, in the case 

 of the earth-crust, the surface which is mapped only reveals the 

 ends and edges of its component rocks; that is, the broken-off 



MN.W 



FIG. 51. Geological section across the Isle of Wight. 



ends of the rock masses, the rest of which plunge into the earth- 

 crust out of sight. The majority of these are stratified rocks, 

 either horizontal or inclined in various ways, but usually according 

 to a regular plan in any one district. The relation of outcrop to 

 structure inside the earth's crust is like that seen on the surface 

 and on the cross-cut of a piece of well-grained wood. Cuts across 

 the rocks indicated in a geological map are often available in 

 railway cuttings or large quarries, and they may be called geolo- 

 gical sections (Fig. 51). Such sections enable us to examine the 

 composition and character of the rocks exposed, and in addition 

 show the order of the strata and their angle of dip. They make 

 clear that those lowest in the sequence must be the oldest, and 

 those resting on their surfaces newer, because each stratum when 

 formed must have had a foundation to rest upon. The results 

 of observations so made are placed on the map, some on the 

 legend or explanation, others upon the map itself. Thus arrows, 



VOL. VI. H 



