14 FIELD GEOLOGY. 



different, it being frequently found that brooks and 

 water-courses work their way back along such planes of 

 separation. 



For the sake of constant reference it is best to have 

 the plain copy of the map, which is to be geologically 

 coloured, cut up into slips of convenient size, say 6 by 

 4J inches. They may be secured by bands within the 

 pages of a note-book or pocket map-case. Six-inch 

 maps must be used in larger sheets carried in cases slung 

 from the shoulder. If the slips are cut all to one size, 

 they can afterwards be mounted on linen to fold in the 

 usual manner. 



General Propositions. The following three proposi- 

 tions, if remembered, will be of material assistance : 



1. The boundary lines of horizontal strata exactly 

 coincide with the contours. 



This must be the case, however uneven the 

 surface of the ground where the outcrop occurs. 



2. The boundary lines of strata dipping towards a 

 hill are less winding than the contours. 



This is evident if we consider that were the 

 dip to be gradually increased until the strata were 

 vertical, the lines of outcrop would gradually ap- 

 proach, and finally become parallel straight lines. 

 Therefore, as the dip into a hill, so the line varies 

 from a contour towards a straight line. 



3. The boundary lines of strata dipping from a hill 

 are more winding than the contours. 



This is just the reverse of Prop. 2, for were 

 the dip increased until equal to that of the sur- 

 face slope, the boundaries would run in parallel 

 lines down the flanks, and until the slope varied 



