HANDBOOK FOR THE DEPARTMENT OP GEOLOGY. 505 



and to which frequent references will be made. A copy of this work 

 is to be found upon the table in the exhibition hall. In the present 

 paper, relatiug only to geognosy, the purely descriptive matter has 

 been given in considerable detail since the arrangement adopted did 

 not sufficiently correspond with that of any available publication 

 to make references in all cases advisable. In the three sections to fol- 

 low it is not anticipated so large a proportion of text will be necessary, 

 since it is proposed to follow more closely the arrangement adopted in 

 the usual text books, and since moreover the objects themselves are 

 more striking in appearance and their individual characteristics more 

 readily apparent. 



GEOLOGY. 



Geology is the science which treats of the earth's crust and its sys- 

 tem of development. Physical geology is that branch of the science 

 which treats of those geological phenomena which are attributable to 

 physical agencies, the term physical being here introduced to distin- 

 guish the department from that of paleontological geology, which treats 

 of the plants and animals which existed in past ages of the globe, but 

 which now occur only as fossils or petrefactions. 



The science of physical geology has been conveniently divided iuto 

 four heads: (1) Geognosy, which treats of the earth's substance, the 

 crust, and its composition j (2) dynamical geology, which treats of 

 the agencies by which changes have been brought about in the com- 

 position and structure of the earth's crust; (3) geotectonic or struc- 

 tural geology, which treats of the structure of this crust, its original 

 condition, and the structural changes which it has since undergone ; 

 and (4) historical or stratigraphic geology, which treats of the order 

 of succession of the rocks without regard to their composition or 

 methods of formation. 



I. — Geognosy. — The materials of the earth's crust. 



The earth's crust is composed of mineral matter in various conditions 

 and stages of consolidation, all of which, whether loose like sand or 

 compact like granite, are included under the general name of rock. 

 In the accompanying collections it has been found most convenient to 

 treat this subject under four distinct heads : (1) The chemical elements 

 constituting rocks. (2) The minerals constituting rocks. (3) The 

 physical properties of rocks, as structure, color, and fracture, and (4) 

 The kinds of rocks. 



(1) The chemical elements constituting rocks. 



Although there are sixty-four elements known, but sixteen occur in 

 any great abundance or form more than an extremely small proportion 



