406 NATURAL HISTORY. 



also found, although not frequently, but those that come 

 to light are of immense size ; and also in this formation 

 belong those singular Saurians whose skeletons remind 

 the observer at once of fish and lizard. The fossil 

 plants found in this formation, of simple structure but 

 immense size, belong mostly to the genera of Equlse- 

 tacea, or horse-tails, ferns, or sedges. The secondary 

 or rock formation is divided into three great portions, 

 namely, the Cretaceous or Chalk, the Jurassic and 

 Trias formation. We will proceed first to examine 

 that assemblage of rocks which were deposited or derived 

 from the interior of the earth at the particular geological 

 epoch in which chalk was deposited, and called 



The Cretaceous or Chalk Formation. This struc- 

 ture, evidently formed by marine agency, is composed of 

 cretaceous matter, or chalk resting on a basis of sandstone. 

 Calcareous Tufa (Spar) or Carbonate of Lime, is a fria- 

 ble, chalk-like substance, of a yellow or grayish-white 

 color ; in some locations found so soft that it is reduced 

 to powder by merely atmospheric influence, but in others 

 it forms a solid limestone, of so firm a structure that it 

 is used for building purposes. Among several varieties 

 we will only, in this place, notice a few. 



Chalk, or White Granular Limestone, is mostly 

 yellowish-white, but is often also found of a reddish hue, 

 and is then of a firmer structure than the commercial 

 article known as chalk. It contains a considerable 

 portion of flint, and forms the white chalk clifls on 

 the shores of England, whence that country receives 

 the name of Albion. Occasional strata of Gypsum 

 (Plaster of Paris) and Rock-salt occur in the cretaceous 

 formation ; the first is found at Segeberg, in Holstein, 

 and at Leineberg, in Hanover, and the last in Catalonia. 



