§20. THE LONDON BASIN. 231 



layers of spheroidal nodules of indurated argillaceous lime- 

 stone, internally traversed by veins of calcareous spar, that 

 radiate from the centre to the circumference. From the 

 appearance of septa or partitions which this character 

 confers, these concretions are called Sept aria : shells 

 and other organic remains frequently form the nuclei of 

 these nodules, which are used in prodigious quantities for 

 cement. Many other argillaceous strata contain concre- 

 tionary spheroids of this kind : beautiful specimens are 

 obtained from the Lias, and Oxford clay, and are cut and 

 polished for tables and other articles. The septaria are 

 commonly disposed in horizontal Hues, and lie at unequal 

 distances from each other. Brilliant sulphuret of iron 

 abounds in the clay, and permeates the septaria and many 

 of the organic remains. Crystallized sulphate of lime, or 

 selenite, is also common in these and other argillaceous 

 strata. The cuttings through Highgate Hill, to form the 

 archway ; the excavations in the Regent's Park ; and, 

 more recently, the tunnels carried through a part of the 

 same ridge of clay at Primrose Hill, in the line of the Bir- 

 mingham railroad ; and the cuttings of other railways, and 

 the explorations, by wells, over the whole area around 

 London, have brought to light such prodigious quantities of 

 organic remains, that the fossils of this deposit are almost 

 universally known. The late Mr. Sowerby called early 

 attention to these productions, the first plate in his Mineral 

 Conchology being devoted to the nautili and other shells 

 of the London basin. Immense numbers of marine shells 

 of extinct species ; crabs, lobsters, and other crustaceans ; 

 teeth of sharks, and of many other genera of fishes ; 

 bones of crocodiles, turtles, serpents and birds ; leaves, 

 fruits, stems of plants, and rolled trunks of trees per- 

 forated by boring shells ;—occur throughout these strata, but 

 are located in greater abundance in some spots than in 

 others. The clay and gravel pits at Woolwich, on the 



