516 



REPORT OF NATIONAL MUSEUM, 1890. 



eralogical composition of a rock, but also, and what is often of more im- 

 portance, its structure and the various changes which have taken place 

 in it siuce its first consolidation. Kocks are not the definite and un- 

 changeable mineral compounds they were once considered, but are rather 

 ever varying aggregates of minerals, which even in themselves undergo 

 structural and chemical changes almost without number. It is a com- 

 mon matter to find rock masses which may have had originally the 

 mineral composition and structure of diabase, but which now are mere 

 aggregates of secondary products such as chlorite, epidote, iron oxides, 

 and kaolin, with perhaps scarcely a trace of the unaltered original con- 

 stituents, yet the rock mass retains its geological identity, and to the 

 naked eye shows little, if any, sign of the changes that have gone on. 

 These and other changes are in part chemical and in part structural or 

 molecular. A very common mineral transformation in basic rocks is 

 that from augite to hornblende. This takes place merely through a 

 molecular readjustment of the particles whereby the augite with its gray 

 or brown colors and rectangular cleavages passes by uralitic stages over 

 into a green hornblende, a mineral of the same chemical composition, but 

 of different chrystallographic form. This transformation in its incom- 

 pleted state is shown in the accompanying fig- 

 ure, in which the central nearly colorless portion 

 with rectangular cleavage represents the origi- 

 nal augite, while the outer dotted portion with 

 cleavage lines cutting at sharp and obtuse an- 

 gles, is the hornblende. This change is due to 

 slow and gradual pressure exerted through 

 unknown periods of time upon the rock masses, 

 and the final result is the production of a rock 

 of entirely different type and structure from 

 that which originally cooled from the molten 

 magma. The change such as above described 

 is well shown in the two specimens of gabbro 

 and gabbro-diorite from near Mount Hope, in 

 Baltimore, Ncs. 36754 and 36755. These are both portions of the same 

 rock mass, but one is a plagioclase hypersthene rock, while the other is 

 a plagioclase hornblende rock; in other words, one is a gabbro while 

 the other is now a diorite, although both are chemically identical and 

 were once mineralogically and structurally identical as well. Another 

 and very common change shown by this method is that from olivine, 

 pyroxene, or other magnesian silicate minerals to serpentine. This 

 change will be dwelt upon more fully in the collection showing the 

 origin of serpentinous rocks. 



This science of microscopic petrography, as it is technically called, 

 has also been productive of equally important results in other lines. 

 As an instance of this may be mentioned the discovery that the struc- 

 tural features of a rock are dependent not upon its chemical composition 



Fig. 90. 



Augite Altering into Horn 



blende. 



(After Hawes. ) 



