Plastic Clap. 



187 



It occupies a projecting point of the cliff, covered at the top, and for 

 a considerable extent upon its sides, with sand and gravel, and exposed 

 to degradation from winds and rains. The edges of the conglomer- 

 ate, however, project through the sand at the extremity of the point 

 where its curved position is obvious ; and on one side alayer of clay 

 appears above the conglomerate. Below these beds the sand conceals 

 the regular strata in the cliff ; but the inclined lines beneath the sand 

 and gravel, in the above sketch show the dip of these strata in the vi- 

 cinity, wherever they are visible ; nor could I doubt but the conglom- 

 erate bed is superior to the clay, and a later formation. And I find 

 that in some of the tertiary strata in Europe, above the plastic clay, 

 occur layers of calcareous, ferruginous, and bony breccia; as in the 

 valley of the Danube, described by Mr. Murchisson.* But on fur- 

 ther examination I found in the vicinity of the bed of conglomerate 

 above described, two or three other beds distinctly interstratified with 

 the clay, and exhibiting the same dip. This rock does, therefore, form 

 a part of the plastic clay formation ; whatever we may think of the 

 case described above. And I cannot find that the like deposite, con- 

 taining the bones of vertebral animals, occurs in any plastic clay for- 

 mation described in Europe; although in England, pebble beds alter- 

 nate irregularly with the sand and clays, but it does not appear that 

 these are consolidated.! 



Another variety of conglomerate at Gay Head, consists of pebbles, 

 chiefly quartz, cemented by a great abundance of the hydrate of iron, 

 and often containing hollow nodules of the same. I am not sure that 

 this alternates with the layers of clay and sand. On account of the 

 great quantity of its debris, that has fallen down upon the face of the 

 cliff, I found it difficult to ascertain its true position in regard to the 

 strata of clay and sand. I was not without a strong suspicion, that 

 it might lie in nearly horizontal strata upon the top of plastic clay ; 

 nay, I was led to enquire whether it might not be diluvium consolida- 

 ted by iron. 



A third kind of conglomerate has been already alluded to, as ex- 

 isting at this cliff, in rolled masses, a few inches in diameter, in the 

 sand stratum. It occurs, also, very commonly along the beach ; hav- 

 ing been probably washed out from the cliff It is unlike any rock 

 that I have ever met with. The nodules are almost if not entirely 



* Philosophical Magazine, March, 1831, Vol. 9. p. 218. 



t De la Bcche's Manual, p. 235 : 2d Edition, London, 1832. 



