CONSTRUCTION OF THE EAKTH. 



137 



Fig. 64. 



west at 20. Then suppose that we find one or two oth- 

 er detached exposures of sandstone and shale to the 

 southwest of the above, with similar strikes and dips. 

 And, lastly, suppose that in the southwest corner of the 

 district we find some more limestone of quite a different 

 kind from that in the northeast corner, but still dipping 

 to the southwest at 20. It will be obvious that these 

 limestones in the southwest corner of the map lie above 

 all the other rocks, while the limestones in the northeast 

 corner rise out from beneath all the rest." 



221. Calculating the Thickness of Strata. In the case 

 just detailed, we have data for arriving at the thickness 

 of this whole set of strata. The manner in which this 

 is done can be explained on Fig. 65 (p. 138), which rep- 

 resents a section of the district at right angles to the 

 strike of the rocks. " Suppose," says Mr. Jukes, " that 

 we wish to sink a pit at the part marked shaft in the 

 map, in search of the coal, and that, before commencing, 

 we wish to know how deep the coal is there, and how 

 far it is to the limestone below it. We take the map, 



