14 The Irish Naturalist. 



members, and the whole appears to form a single series, with 

 clay preponderating below and sand above, and in yet others 

 confusion baffles classification, and reduces the observer to 

 despair. The distinction between the contorted drift and the 

 boulder beds is the least constant ; these so generally pass into 

 each other that if we give them separate names it can only be 

 for our own convenience ; the line between them and the 

 boulder clay, however, can be more easily drawn, and is more 

 persistent. The full value of its significance we may hope to 

 ascertain when investigation has progressed further ; at 

 present our knowledge is surprisingly incomplete. 



The boulder clay extends all the way from the mouth of 

 Bray River to Killiney, but with frequent changes of level and 

 with variable thickness. Sometimes its summit sinks below 

 the level of the beach, sometimes rises 15 to 20 feet above it ; 

 its base is nowhere seen. Though usually red in colour, it is 

 sometimes, especially in its upper portions, black, but even 

 then, wherever water can percolate, as along the numerous 

 cracks which traverse it, the black colour disappears, and is 

 replaced by red ; possibly, therefore, the red colour is not 

 original. 



Irregularly scattered through the clay are numbers of small 

 stones, w T ith here and there a larger one, sometimes gigantic by 

 comparison, as in the case of one granite block which measured 

 3 feet in length. The stones are usually irregular in shape, and 

 irregularly scratched, but not seldom a boulder with flattened 

 faces and longitudinal striae is to be seen. They commonly 

 consist of the limestone of the district, though, as Dr. Scouler 

 pointed out long ago, travelled boulders from the far north are 

 frequently met with ; thus, pieces of the hard chalk of Antrim, 

 beautifully polished and finely scratched, are far from rare. 

 That these are truly Antrim chalk may be proved by examining 

 thin slices under the microscope, when the characteristic 

 structure of this rock is seen in all its details. 



But what is most remarkable about the boulder clay is its 

 richness in shells and fragments of shells. The fragments are 

 sometimes rounded, sometimes polished as perfectly as if by 

 the hand of a lapidary, and sometimes scratched. The entire 

 shells are either univalves or single valves of bivalves, most 

 commonly of the northern species Tcllina balthica ; one speci- 

 men of Tcllina was found with its natural colour as fresh as 



