524 REPORT OF NATIONAL MUSEUM, 1890. 



ing the working properties of the stones, are dwelt upon more fully in 

 the hand-book relating to the collection of building and ornamental 

 stones. 



(4) The kinds of rocks. 



In the present transitional state of our knowledge regarding the 

 chemical and mineralogical composition of rocks, their structural fea- 

 tures, and methods of origin, no scheme of classification can be advanced 

 that will prove satisfactory in all its details. The older systems which 

 were made to answer before the introduction of the microscope into geo- 

 logical science are now found to be founded upon what were in part false, 

 and what have proven to be wholly inadequate data. This is especially 

 true in regard to eruptive rocks. The time that has elapsed since this 

 introduction has been too short for the evolution of a perfectly satis- 

 factory system ; many have been i>roposed, but all have been found 

 lacking in some essential particular. To enter upon a discussion of the 

 merits and demerits of the various schemes would obviously be out of 

 place here, and the student is referred to the published writings of Nau- 

 mann, Senft, Yon Cotta, Eichtofen, Vogelsang, Zirkel, Eosenbusch, and 

 Geikie, as well as those of the American geologists Dana* and Wads- 

 worth.f In the scheme here presented the Curator has aimed to simplify 

 matters as much as possible, and has not hesitated to adopt or reject 

 any such portions of systems proposed by others as have seemed desir- 

 able. 



All the rocks forming any essential part of the earth's crust are here 

 grouped under four main heads, the distinctions beiug based upon their 

 origin and structure. Each of the main divisions is again divided into 

 groups or families, the distinctions being based mainly upon mineral 

 and chemical composition, structure, and mode of occurrence. We 

 thus have : 



I. Aqueous rocks. — Eock-s formed mainly through the agency of 

 water as (A) chemical precipitates or as (B) sedimentary beds. Having 

 one or many essential constituents ; in structure laminated or bedded; 

 crystalline, colloidal or fragmental, never glassy. 



II. JEolian rocks. — Eocks formed from wind-drifted materials. In 

 structure irregularly bedded ; fragmental. 



III. Metamorphic rocks. — Eocks changed from their original condi- 

 tion through dynamic or chemical agencies, aud Avhich may have been 

 in part of aqueous and in part of igneous origin. Having one or many 

 essential constituents. In structure bedded, schistose, or foliated. 



IV. Igneous rocks. Eruptive. — Rocks which have been brought up 

 from below in a molten condition, and which owe their present struc- 

 tural peculiarities to variations in conditions of solidification and coin- 



* On some points in Lithology, Am. Jour. Sci., vol. xvi, 1878, pp. 335 and 431. 

 t On the classification of rocks. Bull. Mus. Cornp. ZooL Howard College, No. 13, vol. 

 V.; also Lithological Studies. 



