DESCRIPTIVE GEOLOGY: GENERAL CONSIDERATIONS. 



26l 



sections a mean or average section be calculated, we shall find that 

 each formation in the resulting mean section occupies or spreads 

 through a definite vertical range. Similarly, if for any other zone 

 another mean vertical section be calculated, this resulting section 

 will be found to differ from the previous one in the relative levels or 

 vertical ranges of each of the composing formations. In fig. 53, 

 diagrams a, b, c, d % represent four such mean vertical sections, de- 



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_ IT 



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< r_ 



1 



1 





it' 







6 



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1 i'i : 



4 

















% 









2 



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I 



6 





5 





4 











Fig- 53. 

 S=Murree beds. 

 7 ' = Kuldana series. 

 6=Nummulitic limestone. 

 5 ■ Jura-Cretaceous. 

 4= Trias. 

 3 = Infra-Trias. 

 2 = Slate series. 

 i=Crystalline schists, etc. 



duced respectively for the zones A, B, C, and D. With reference to 

 their construction which here has been chiefly of a graphical kind, 

 it may be remarked that greater accuracy in the resulting mean posi- 

 tion of each formation might be obtained if necessary, and the great- 

 est possible accuracy if a large map showing true contours were 

 available. The general principles involved are, however, independent 

 of such extreme accuracy. 



For the sections a, b f c, and d, a very simple law is seen to hold 

 good, namely, that the mean level of a formation rises as we proceed 

 through the sections from d to a ) or sinks as we proceed in the 

 opposite direction. For instance, following out the Nummulitic 

 formation we see that in d it occupies the base of the section, in 

 c a middle position, in b an uppermost position, whilst in a it is 

 unrepresented. Similarly, following the Infra-Trias formation, it is 

 at the top of the section in a> it occupies a middle position in b> 

 and is lost to sight in c : and so on for any formation. 



( 261 ) 



