208 PETRIFACTIONS AND THEIR TEACHINGS. CHAP. III. 



and relations of the rocks and strata in which the most impor- 

 tant fossils were discovered. 



GEOLOGICAL STRUCTURE OF THE SOUTH-EAST OF ENGLAND. 

 The various strata that appear on the surface of the South- 

 Eastern part of our island, constitute four natural groups or 

 formations, namely, 1. The Drift, or Diluvium; 2. The 

 Eocene, or Tertiary strata ; 3. The Chalk formation ; 4. The 

 Wealden. 



The Drift, or Diluvium. These are the superficial accumu- 

 lations which are irregularly spread over the surface, and form 

 the immediate subsoil. They consist of layers of loam, clay, 

 sand, gravel, and boulders, of variable extent and thickness, 

 composed of waterworn materials which bear unequivocal 

 evidence of having been drifted or transported from a distance 

 by floods or inundations, or other agents of a like nature. 

 Over considerable areas, as, for example, on both sides of the 

 valley of the Thames, a fine silt or mud appears to have been 

 deposited upon the drifted boulder-clay since the country 

 had acquired its present physical configuration ; and in this 

 deposit, as well as in the ancient drift beneath, are found 

 skeletons and detached bones and teeth of extinct species of 

 Elephant, Rhinoceros, Hippopotamus, Elk, Ox, Horse, &c. ; 

 and, in some places, the remains of species still existing, are 

 associated with those no longer known. Bones of carnivora, 

 as the Hysena, Bear, Tiger, Wolf, also occur in the drift, as 

 well as in the ossiferous caves of which we shall treat when 

 describing the contents of another department of the Museum. 



The Tertiary or Eocene Strata of London. The metropolis 

 of England, like that of France, is built on the surface of a 

 tertiary formation, consisting of beds of blue, yellow, and 

 mottled clays, several hundred feet in thickness, with inter- 

 calations of loam, sand, gravel, &c. Some of the strata 

 abound in extinct species and genera of marine shells, fishes, 

 Crustacea, &c. 



The Chalk Formation. The white calcareous rock of this 

 formation is well known by the name of Chalk; but in the 

 nomenclature of Geology, the term Chalk Formation comprises 

 not only the pure limestone that extends over a considerable 

 part of the country, but likewise subordinate strata of sand 

 and sandstone, clays, marls, and limestones, very dissimilar in 

 appearance and mineralogical character, but which present 



