1865.] BEYCE AREAN DRIFT- BEDS. 205 



I satisfied myself that the shells lay on a particular horizon, and 

 that the drifts were capable of a clear division. A second visit was 

 paid to these beds early in June, when additional shells were found, 

 and the. entire series again most carefuUy examined*. 



In conducting this examination, the object of enlarging the list 

 of fossils was held quite subordinate to that of working out the 

 natural succession of the beds, and establishing definite relations 

 among them. It has long been common, and the practice still 

 prevails to some extent, to overlook the distinctions among the 

 Drift-beds and to class the whole, under the term " Boulder-clay " 

 or " Till," as one formation. There can be little doubt, however, 

 that if the beds of each particular district were carefully examined, 

 an order of succession marked by distinct characters would be 

 found in most cases to exist. But the subject is difiicult, and the 

 appearances misleading ; causes of error present themselves which 

 are not met with in the case of rocky strata. These must now 

 be briefly stated. 



The upper layers under the surface-soil are of loose texture and 

 pervaded hj fissures, while the lower layers, consisting of dense 

 clay, are retentive of water. Thus a great hydrostatic pressure 

 comes to be exercised ; and since the beds have not that coherence 

 which enables one part to lend support to another, a slip takes place. 

 The same effect is produced by the undermining of a stream, 

 or the action of waves and tides. The order of the beds is thus 

 completely masked ; a bed high in a bank or cliff may by such a 

 landslip be placed in front of the lowest bed, and so remain for a 

 long time, until by successive slips and undermining the whole is 

 washed away and the natural order of the beds is once more 

 revealed. If taking place throughout the whole height of the bank, 

 it will often happen that such a slip will show itself by inequalities 

 along the grassy brows at the summit ; but in many cases there is 

 no such evidence of its occurrence, and the slip can only be de- 

 tected by careful searching and an experienced eye. There is, 

 beside all this, the action, in the exposed surface of such a vertical 

 section, of the sun and air, the beating of rain, and trickling of 

 runlets of water : these produce a general wash-over, which gives 

 the same facing to all the beds, and completely obscures the natural 

 order of succession. The wash renders it impossible to see the line 

 of junction, and it is often necessary to search over a large surface. 

 One finds a hammer almost useless among such beds ; a pickaxe and 

 spade are needed for opening up the beds, and clearing off debris ; 

 but these are weapons which the geologist can seldoni carry to the 

 field ; yet "wdthout them he wiU find it impossible to clear off the 

 planes of junction so as to expose the true succession. 



The Arran sheU-beds are most easily visited from the hotel at 

 Lag, near the south end of the island. They are well exhibited in 



* On the second visit, I had the pleasure of being accompanied by the Rev. 

 H. W. Crosskey, of Glasgow, one of the most zealous and successfid la- 

 bourers among the newer Tertiaries of Clydesdale since Mr. Smith, of Jordan- 

 hill, ceased to work actively among them. 



