MEANS OF DISTINGUISHING FORMATIONS. 183 



3d. The jura'ssic, embracing the lia'ssic, the o'olite, and vvealden groups : 

 4th. The cretaceous, embracing the lower greensand, gault, upper green- 

 Band, chalk marl, chalk without, and chalk with flints. 



7. Fourth. The tertiary formations, which, being more re 

 cent, covered all the preceding formations; they date from a period 

 when animals and plants belonging to all the great classes existed, 

 but still anterior to the creation of man : 



The tertiaries are subdivided into three groups : 



1st. The older tertiary or eocene, which embraces the London clay, bag- 

 shot sand, and Paris Basin. 



2d. The middle tertiary, or miocene, which embraces the Coralline crag, 

 Red crag, the Molasse of Switzerland, &c. 



3d. The newer tertiary, or Pliocene, which embraces Norwich crag, the 

 Bub-Apennine beds, the Brown coal of Germany, &c., as well as the super- 

 ficial deposits, called Pleistocene, consisting of diluvium and alluvium. 



8. Fifth. The modern formations, which are contemporaneous 

 with the existence of man on the earth, and are still being formed. 



The subdivisions embrace : 



1st. Peat-bogs, formed by the accumulation of the debris of certain plants. 



2d. Coral formations, from the multiplication of polypa'ria as seen in the 

 coral islands of the Pacific. 



3d. Concretionary formations, formed by calcareous and other matters, 

 found in solution in the waters of certain springs, &c. ; as travertin, stala'c- 

 tites, stala'gmites, &c. 



4th. Formations from transport or drift ; as fluviatile, terrestrial, or marine 

 alluvions, dunes, &,c. 



5th. Humus, or vegetable earth, formed directly by the disintegration of 

 other formations, and their mixture with the products of decomposition of 

 plants and animals, spread in a layer of more or less thickness, on almost 

 every point of the surface of the earth. 



9. All these deposits are superposed one on the other, in a con- 

 stant order; and if it were possible to make a sufficient section in 

 a part of the globe where they all exist together, we should find a 

 succession of twenty-seven stories, or layers, distinguishable by 

 their different characters. But each of the great deposits is divided 

 and subdivided into various layers, more or ]e?s distinct, composed 

 most frequently of arena'ceous substances, clay and limestone, of 

 different degrees of consistence, and in beds of varying thickness. 

 The assemblage of their alternate beds often forms successive layers, 

 several hundred yards thick. 



10. It is evident, that if such sections existed in the crust of the 

 earth, we could see all the beds, and easily distinguish their rek- 



7. From what period do the tertiary formations date 1 What are the 

 divisions of the tertiaries 1 



8. From what period do the modern formations date 1 What formations 

 are embraced in the divisions of the modern formations 1 How is humus 

 *brmed 1 



9. What is the r Tangement of the several deposits composing the crust 

 of the earth 1 



10. Why is it difficult to distinguish the relative ages of deposits? 



