396 IDIOPHTSICS. 



in geological classifications, was first brought into view by Mr. 

 Smith of England, about the year 1793 : but it was more fully 

 appreciated in the great work of Cuvier and Brongniart, on the 

 Environs of Paris, published in 1811; and further illustrated in 

 Cuvier's work on Fossil Bones, in the following year. 



The aqueous, or Neptunian theory, of Thales, was revived, in 

 1740, by De Maillet of France, in a work entitled Telliamed : and 

 this opinion was adopted by Linnaeus, and Werner. Werner carried 

 this theory so far as to maintain that all superficial or superincumbent 

 rocks were formed by deposition in water : but the opposite, or Vul- 

 canian theory, that basaltic and trap rocks, though lying on the 

 earth's surface, are of igneous origin, having been cooled from a 

 melted state, was revived by Hutton, about the year 1785 ; and 

 afterwards maintained by Playfair; and finally admitted by all parties 

 to be correct. The theories published by Burnet in 1681, and by 

 Whiston in 1722, were merely crude speculations. 



Leibnitz and Buffon maintained that the earth was originally in a 

 state of igneous fusion ; and that its whole interior is still in a melted 

 state, owing to the intense central heat. Mr. Lyell, has discarded 

 the doctrine of central heat, in his Principles of Geology, published 

 in 1830, in which he attempts to explain the former changes of the 

 earth's surface, by causes now in action : a doctrine which, in a qua- 

 lified sense, may doubtless be true. Dolomieu was one of the first 

 geologists who studied carefully the nature of active volcanoes ; and 

 new light was thrown on this subject by the researches of Hum- 

 boldt in Central America, 1799-1804. Saussure, in his travels, 

 made many important geological observations ; as did also Professor 

 Pallas, who examined the minerals and fossils of Russia and Siberia. 

 The geology of Great Britain has been extensively illustrated by the 

 labors of Jameson, Conybeare, Phillips, Sedgwick, Buckland, Mur- 

 chison, De la Beche, and others already named ; as that of France 

 by Brochant de Villiers, Elie de Beaumont, Dufrenoy, and others. 

 Our own country is at present the theatre of extensive geological 

 explorations, by zealous and competent observers ; but we have only 

 room to cite the names of Maclure, Silliman, and Eaton, as pioneers 

 of American geology; and of Prof. Hitchcock, Dr. C. T. Jackson, 

 Professors H. D. and W. B. Rogers, and W. W. Mather Esq., as 

 gentlemen charged with important geological state-surveys. 



We proceed to give an outline of this science, under the heads of 

 Introductory, Systematic, Descriptive, and Physical Geology. 



1. Under the head of Introductory Geology, belongs a descrip- 

 tion of the different rocks which compose our globe, so far as they 

 are accessible to examination. The term rock, is applied by geolo- 

 gists, not only to the hard masses usually so called, but also to the 

 various soft or pulverulent substances, derived from the former, by 

 disintegration, or pulverization, and commonly called earths. The 

 hard rocks, are either simple minerals, such as are studied under 

 Mineralogy; or composed of two or more simple minerals, united by 

 aggregation. Among the minerals which occur in large masses, and 

 hence may be called simple rocks, are quartz, feldspar, hornblende, 

 serpentine, gypsum, and limestone, or marble. 



