STRATA IN THE NEIGHUOURBOOD OF LONDON. 4^ 



gray, veri^ing on black. It is not only characterised by 

 these circumstances, but by the numerous septaria, which 

 are dispersed through it, and by the peculiar fossils, which 

 it contains. 



The difference of colour observed between its superior and Cause of the 

 inferior part, and which has generally been supposed to be f^'^erencein 

 owing to a difference in the degree of oxidation of the iron 

 present in it, appears to be the result of a difference in the 

 quantity of it, occasioned by the washing away of this metal 

 in the upper part by the water which percolates through it, 

 and which runs off laterally by the numerous drains made 

 near the surface. The dark red colour of tiles made from the 

 blue clay, the reddish-yellow colour of thep/ace bricks made 

 of the yellowish-brown clay, and the bright yellow hue of 

 the washed malms, those bricks which are formed of the yel- 

 low clay which has been exposed to repeated washings, are 

 thus accounted for. 



The septaria lie horizontally, and are disposed at unequal Septaria. 

 distances from each other in seemingly regular layers ; and, 

 as has been just observed of the stratum itself, they become 

 of a paler colour, and it may be added suffer decomposition, 

 when placed so high in the stratum, as to be exposed to the 

 action of percolating water. They frequently include por- 

 tions of wood pierced by the teredines, nautili, and other Their septa 

 shells ; and it is a fact, that may be worthy of being attended frequently jn« 

 to, while inquiring into their formation, that the septa ofsybsunces 

 calcareous spar frequently intersect the substances enclosed enclosed in 



xi i • them. 



m the septaria. 



This stratum is to be found not only wherever the pre- Extent f tl "« 

 ceding deposition extends, but in other parts also, where that stratum. 

 has been removed. The cliffs of this clay, at Shepey, ex- 

 tend about six miles in length; the more elevated parts, 

 which are about ninety feet in height, being about four 

 miles in length, and declining gradually as they terminate 

 towards the east and west. 



Xhe fossils of this stratum have been already carefully Its fossils, In 

 particularised. A catalogue of those found at Shepey was ^^'^P^J> 

 added by Mr. Jacobs to his Plantce Favershamienses : and an 

 account of several of the fossil fruits found at Shepey was 

 published by Dr. Persons in the-fiftieth volume of the Phi- 



VoL. XXXI.— Jan. Is 12. E losophical 



