i6o 



THE BOOK OF NATURE STUDY 



and coal workings will probably show on which side this occurs, the 

 workings being carried down from the surface until they meet the 

 coal below. Working then proceeds on all sides from the bottom of 

 the shaft, and as much of the seam as it is safe to remove is taken 

 out and brought to the surface. Other strata of rock will have 

 been pierced by the coal shaft before reaching the coal, and they 

 will come to the surface between the outcrop of the coal seam and 

 the pit's mouth (Fig. 49). Still other rocks will be met with below 

 if the shaft pierces through the coal seam, and they in their turn 

 will outcrop beyond the coal crop. This will be true whether 



:V;|Oligocne 

 Eoceno 

 Chalk 



I Gaul t 



Lower 

 Greenaand 



Waaldan 

 Beds 



FIG. 50. Geological Map of the Isle of Wight. 



the beds of rock are horizontal or inclined. If inclined, the bed 

 is said to dip at so many degrees to the horizontal, and the direction 

 of dip is measured along the line down which water would run on 

 its surface. This direction is observed by its compass-bearing, 

 and a knowledge of it is vital if we wish to understand the rock 

 sequence and relations at any locality. 



The line drawn at right angles to the dip, as thus defined, along 

 the surface of the bed will always be found to be horizontal, and 

 a special name is given to it, the strike of the stratum. It will, 

 of course, be a contour line of the bed. It is easy to see that if 

 the rocks are dipping and the ground is horizontal the outcrop 



