EXCURSION XV. 183 



beds is completely masked; a bed ten, twenty, thirty feet 

 higher in the series is placed in front of perhaps the lowest 

 bed, and so remains for a long period, till by successive slips 

 or underminings the whole fallen mass is removed, and the 

 natural order of the beds is again revealed. In most cases 

 satisfactory evidence that such landslips have taken place 

 will be had by examining the grassy brows along the summit 

 of the cliffs. Then again, the beating of rain, the action of 

 the air and sun, and the trickling of runlets of water over 

 the front of such a section, produce a sort of general wash- 

 over, as we may call it, which gives the same facing to all 

 the beds, and completely obscures the natural succession. 

 There is a third obstacle and cause of error, like what one 

 sometimes meets with in the case of the rocky strata the 

 difficulty of finding junctions. The wash renders it impos- 

 sible to see the line of contact, and it is often necessary to 

 search over a large surface. The observer takes the field 

 armed with a hammer only, but finds his weapon, good for 

 other purposes, of no use in this contest. A pickaxe and 

 spade are required for opening up the beds, and clearing 

 away loose matter, in order to obtain a distinct view of the 

 planes of contact. 



The difficulty of finding a true section, and so determining 

 the actual succession of the beds, is now apparent; and the 

 great probability, that, under the name of boulder-clay, beds 

 really distinct have been included. And we cannot help 

 thinking that much of the confusion which prevails has 

 arisen from overlooking the causes of error now adverted 

 to. One of the sections under consideration was visited by 

 us three times, and another twice before we were able to 

 satisfy ourselves about the order of the beds. In several 

 places we left the sections completely opened up for the 

 benefit of future observers : but the elements of change may 

 soon alter the features. 



86. The base of the section in the Cloinid burn is the 

 true old boulder-clay resting on the upturned edges of the 



