28 FIELD GEOLOGY. 



one map, although for the sake of clearness they have 

 been hitherto treated as distinct : on placing them to- 

 gether in proper position they would be found the same, 

 but on a larger scale, as the coloured map in the frontis- 

 piece. The former may be considered as one's working 

 or field copies, the latter as completed for reference or 

 for publication. 



One slip or one area will frequently elucidate what is 

 .somewhat obscure in another ; as the Upper Green-sand, 

 mapped in either figure 6 or 7, prove that the sandy soil 

 at the S.E. corner of figure 5 is due to that formation, 

 although in the slip itself no sections were obtainable. 



Note. The process of mapping the older and 

 more disturbed rocks is somewhat different, and 

 is much facilitated by an acquaintance with the 

 methods of observing their inclination and litho- 

 logical characters. An example of this kind of 

 work will be given, but is deferred until after 

 those methods have been considered in Parts II. 

 and III. 



Memoranda. We have now constructed a geological 

 map on which are shewn (with, let us hope, a reasonable 

 approach to accuracy) the outcrops of four formations 

 the mottled Loams, the Chalk, the Upper Green-sand, and 

 the Gault. We have proved their relation to each other, 

 and with the data obtained in the pits shall be able to 

 draw therefrom a section also, perhaps several hundred 

 feet in thickness. But, as this will be considered in the 

 second portion of the work, we will now proceed to no- 

 tice several things which should be borne in mind while 

 mapping geological boundaries. 



It frequently happens that lines have to be drawn 



