GEOLOGY IN NATIONAL MUSEUM MERRILL. 277 



constituent parts are grouped together — the specific gravity, the 

 chemical composition, color, and fracture, and manner of breaking of 

 rocks, each feature of which is illustrated by one or more specimens. 

 It is common to divide these structural features into two classes ac- 

 cording as to whether or not they are visible by the unaided eye or 

 only by a magnifying power. The first are known as the macroscopic 

 or megascopic, the second the microscopic structures. The micro- 

 scopic structures are made visible by means of thin sections suffi- 

 ciently transparent to be studied under the microscope. A series of 

 these sections is shown, and also, in the transparencies in the win- 

 dows, the appearance of certain portions as seen under a high 

 magnifying power. 



Other series illustrate the specific gravity, chemical composition, 

 and color of rocks, and, so far as possible, explain the agencies to 

 which these characteristics are due. 



(4) The Kinds of Rocks {Petrology). — These preliminary series 

 are followed by one much more extensive, including all the more im- 

 portant types of rock masses. These are shown mainly in the form of 

 what are known as hand specimens, approximately 3 by 4 inches in 

 diameter, and 1 to 1£ inches in thickness. They are grouped under 

 four main heads, the distinctions being 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. The col- 

 lection as at present exhibited comprises some 1,700 specimens, 

 grouped as follows: 



I. Aqueous rocks. — Rocks formed mainly through the agency of 

 water as (a) chemical precipitates or as (h) sedimentary beds. Hav- 

 ing one or more essential constituents; in structure laminated or 

 bedded ; crystalline, colloidal or fragmental, never glassy. 



II. Aeolian rocks. — Rocks formed from wind-drifted materials. 

 In structure irregularly bedded ; fragmental. 



III. MetamorpMc rocks. — Rocks changed from their original con- 

 dition through dynamic or chemical agencies, and which 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 

 structural peculiarities to variations in conditions of solidification 

 and composition, having, as a rule, two or more essential constitu- 

 ents. In structure massive, crystalline, felsitic or glassy, or, in cer- 

 tain altered forms, colloidal. 



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