THE REVOLUTIONS OF THE CRUST OF THE EARTH. 327 



should be argillaceous and impermeable, but never sandy nor gravelly.* 

 Peat is principally composed of the ligneous and fibrous parts of the Carex 

 erio2)horum, and certain mosses, whose decomposition has been retarded 

 by the circumstance that the temperature of the water in which they 

 were, never passed beyond a certain mean, and also by the presence of 

 some organic acids. The depth of peat-bogs is often very great, some- 

 times as much as 18 yards. Their lateral extent is seldom over 3 yards, 

 ordinarily not more than 1. 



M. Lesquereux has made upon the subject of the geographical distri- 

 bution of peat-bogs, some very important remarks. He observed that 

 the peat-bogs of Europe extended between the forty-fifth or the forty- 

 sixth degree of north latitude and the regions where the growth of 

 ligneous plants ceases. A corresponding limitation is given for their 

 vertical extension; in the Alps, for example, they are contained in the 

 valleys up to a height of 2,600 meters (8,500 feet). According to Mr. 

 Darwin, there is no peat in the island Chiloe at 41° to 42° of south lat- 

 itude, notwithstanding the great number of its marshes ; on the con- 

 trary it is very abundant in the Obonos Islands in 45° latitude. The 

 temperature favorable to the development of peat-bogs is an annual 

 mean of 5° to 8° C; the development ceases when the temperature at- 

 tains 10°. M. Lesquereux concludes from this that the temperature of 

 the globe has not sensibly changed since the Quaternary period, for if 

 during this period the temperature of Scotland had fallen to that of 

 the Azores, we would find somewhere in the south of Europe deposits 

 contemporary with this cooling, which is not the case. M. d'Archiac 

 however, considered that this supposed decrease of temperature is not 

 incompatible with the absence of peat, for he thinks that this low de- 

 gree of heat may have continued for too short a time to produce great 

 deposits ; or, if these were formed, they may have been carried away by 

 the diluvian action which denuded this zone.t As to the age of the 

 recent peat-bogs, there is but one opinion, that their formation com- 

 menced with the cessation of the last diluvian phenomena. 



Animals, more than plants, contribute to the formation of permanent 

 crusts at the surface of the globe ; especially the inferior animals, the 

 Crustacea^ the Molliisca, Cephalopoda, Gasteropoda, Acephalay Brachyopoda, 

 the Badiata, Echini, Crinoidea, Asterioida, the Polyps, the Foraminifera, 

 and the Infusoria, which are born, live, die, and are buried in the same 

 place, upon the same bank, at the foot of the same rock, and which form 

 thick strata, consisting entirely of their calcareous remains. The residue 

 of the terrestrial animals is less abundant, because their remains are 

 generally disorganized by the influence of atmospheric agents before 

 they are carried into the sea. Still, the tusks of the mammoth found in 

 the ice of Siberia, the bones discovered in the caverns, the guano of the 

 Chincha Islands of Peru, of the coast of Chili, and of Terra del Fuego, 



*D'Archiac, Cours de Paliont. Strut., t. ii, p. 390. 

 t D'Archiac, op. cit., t. ii,p. 397. 



