232 THE WONDERS OF GEOLOGY. Lect. III. 



banks of the Thames, are full of fresh-water univalve 

 shells ; and at Plumstead, Bexley, and other places, marine 

 bivalves occur in clay, and in indurated argillaceous lime- 

 stone.* 



21. The Isle of Sheppey. — The Isle of Sheppey is 

 entirely composed of the London clay, and the thickness of 

 the beds is upwards of 550 feet. It has long been celebrated 

 for its organic remains ; and I may observe, that the disco- 

 very of seed-vessels and stems of plants in pyritous clay, in a 

 visit which I made to Queenborough, when a youth, tended 

 to confirm my early taste for geological researches. The cliffs 

 on the north of the island are about 200 feet high, and consist 

 of clay abounding in septaria, which are washed out of the 

 cliffs by the action of the sea, and collected for cement. 

 The organic remains are, however, unfortunately so strongly 

 impregnated with pyrites, that the collector often finds the 

 choicest fossil fruits in his cabinet, like the fabled apples of 

 the Dead Sea, one moment perfect and brilliant, and the next 

 decomposed and fallen to pieces, leaving only an efflorescent 

 sulphate of iron. A solution of isinglass in spirits of wine, 

 applied as a varnish, is one of the best preservatives. The 

 same species of animal "and vegetable remains that are 

 found in the blue clay of the metropolis, are also met with 

 in profusion in the Isle of Sheppey. f 



Fruits, seed-vessels, with stems and branches of trees, of 

 a tropical character, probably drifted by currents into the 

 gulf of the London basin, occur in such abundance and 

 variety in the Isle of Sheppey, that the existence of a group 

 of spice islands at no great distance, seems necessary to ac- 

 count for so vast an accumulation of vegetable productions. 

 The seed-vessels are referable to several hundred species ; 

 some are related to the Cardamom, Date, Areca, Cocoa, 



* Messrs. Wethcrcll and Edwards have fine collections of the 

 Highgate fossils. 



f Sec Medals ot Creation, vol. i. p. 175. 



