e left ide, the same beds are recognisable, 

 following and dipping under each other at corresponding 



iK. In uihi-r w.-r.U, the order of succession it 

 found to be the tame in the different streams. Bed A 



.1 B by bed C. 



and so on therefore, where a blank space occurs, 



^ to some surface accumulation, a particular 

 bed may nut IK? viMt.K- m one ! the lines of sc< 

 we can be tolerably sure of the place where, judging 

 from the strata above and below, it would be seen if it 

 came to the surface. We do not hesitate, thcref< 

 draw dotted lines across that place to indicate our belief. 

 A geological map is thus derived partly from what is 

 seen, and partly from what can be legitimately inferred. 



I would further direct attention to the fact that while 



the order in which the beds occur remains the same in 



all the streams upon our n. .27 and 28), the 



spaces between them van* considerably. This difference 



may arise from one or other of three causes ; citl 



variation in angle of dip, or (2) variation in thickness of 



strata, or (3) inequalities in the level of the ground. 



tve already considered the effect of a decrease of 



it ion in increasing the breadth of a stratum or series 



ua at the surface of the ground. It is further 



: that if the mass of strata between two known beds 



should swell out or diminish, the breadth of the space 



between their res|>ertive outcrops must correspondingly 



vary. Inequalities of the surface must likewise influence, 



as we have seen, not only the direction of the outcrops, 



.so their breadth. Where, therefore, the angle of 



dip does not change, and the surface of the ground 



