MAPPING. 



from feature must generally be depended on for deter- 

 mining the order of their super-position. Where no 

 sections shewing junctions are met with, the form of the 

 ground may afford some hints as to which of two beds 

 is the upper, and which the lower, for a gravel overlying 

 clay will not make quite the same kind of feature as it 

 does when passing beneath. Assuming of course that 

 " Drift " of some kind is in question, it will generally be 

 found that a rounded hill consists of clay ; if the lower 

 part of its flank be gravel or sand and comparatively 

 steep, such gravel or sand may reasonably be supposed 

 to pass imder the clay but if, on the contrary, it forms 

 a flat or sloping plain, it probably rests upon it. ;; For 

 the lower part of a bed of gravel or sand, protected above 

 by a more tenacious bed of clay, is cut back at a rate dis- 

 proportionate to that of its upper part, and a more or less 

 abrupt rise is the result. Gravels frequently make flats 

 and sometimes long ridges, cut off as it were from the 

 land at similar heights around them. 



The boundary lines of drift must be closely followed, 

 if an accurate map of the beds be desired contour is of 

 but little -use except as a guide to the eye in drawing 

 lines between the points through which they have been 

 found to run, and these points should be at no great dis- 

 tance from each other. A drift clay may be found in 

 one place on the top of a ML, and not on its flank or at 

 its foot, but a few hundred yards away it may perhaps 

 be discovered trailing down the slope, and even crossing 

 the valley, with none on the high ground whatever- 

 This must necessarily be the case, as we see on reflection, 

 for the drifts repose on an irregular surface, and those 

 portions which occupy the old hollows and channels 



32 



