12 Prof. BucKLAND and Mr. De la Beche on the 



sometimes entirely occupies the place of the clay between the limestone beds 

 of this formation. One of the most important points in the geological history of 

 the Purbeck series,, is the occurrence of a bed of oyster-shells called the "cinder 

 bed/' often many feet in thickness^ and almost wholly composed of dark- 

 coloured, small oyster-shells in the midst of a series of strata, some of which 

 contain exclusively shells of freshwater formation, and others an admixture of 

 freshwater shells with those which are marine: this circumstance has been duly 

 noticed by Mr. Webster and Dr. Fitton ; and although we cannot infer from 

 it the return of the sea for any long period in the middle of the Purbeck for- 

 mation, yet it shows that the district it occupies could not have been a lake of 

 pure fresh water, but was probably an estuary at the time when these oysters 

 occupied its bottom, and were accumulated to the thickness of many feet 

 over a distance of many miles*. 



We are disposed to agree with Mr. Webster, and to adopt more confidently 

 than he has done the opinion he advances with respect to the propriety of 

 referring to the lower region of the Purbeck series the beds of white cal- 

 careous slate, usually destitute of organic remains, which occur between the 

 undoubted Purbeck beds and the Portland stone ; these beds resemble the 

 compact varieties of Purbeck stone, which are devoid of shells, and which 

 near Lulworth attain a thickness of from 60 to 100 feet. In Portland the 



* The following description of the actual state of the lake Menzale at the mouth of the Nile, by 

 a modern traveller, is highly illustrative of the mode in which living animals of a mixed character 

 are associated together near the confluence of great rivers with the sea. 



" The lake Menzale is only five miles distant from Damietta. I should judge it to be sixty-five 

 miles long, and fifteen broad: it is not, properly speaking, a maritime lake, but formed by the 

 increase of the Damietta branch of the Nile ; the depth is from three to five feet, and on pushing 

 an oar to the bottom I have observed it coated with the common mud of the Nile for about twenty 

 inches deep. Along all the length of the lake a narrow tongue of land separates it from the sea, 

 but not wholly : there are four passages through which it is possible for barks to sail ; through 

 two of them 1 passed with a good deal of difficulty. At the mouth of each there is a bar of sand, 

 which makes the passage perilous from the sea. No sea or lake in the world can perhaps boast 

 of the same quantity of fish in a given space as the lake of Menzale. 



" The principal sorts of fish caught here are the Perca Lutk, or Lot's perch ; another species called 

 kescher ; the charamoot or Silurus anguillaris, — the fin is said to be poisonous ; the hurra, or red 

 mullet ; the kelp el bahr, or sea-dog ; the casheff, or Mormyrus anguilloides of Linnaeus ; this I 

 have seen weighing thirty pounds. The salmon of the Nile is found in the upper part o^ Menzale 

 weighing from 80 to 100 pounds. The mixture of sea and river water in the lake causes it to be 

 neither salt nor sweet ; so that both river and sea fish are to be found here in equal quantity: but 

 both, in my opinion, of an inferior quality. The quantity of birds which cover the lake is pro- 

 digious. Pelicans, cormorants, cranes, and herons live only on the fish." — Madden' s Travels in 

 Egi/pt, <^c. vol. ii. p. 171 — 175. 



