154 The American Geologist. September, looi. 
GABBRO-DIORYTE ( METAGABBRO.) * 
Like the saussnrite gabbros these rocks are formed by the 
metamorphic changes which have gone on in the hypersthene- 
p-abbros, but in this case only the pyroxene has been altered, 
the greater portion of the feldspar remaining unchanged. 
There does not appear to have been a very extensive develop- 
ment of the gabbro-dioryte in this area and consequently the 
rock is not very abundant. Of the fevy localities where it was 
observed the best is in the vicinity of the Mount Hope church, 
one and one-half miles northeast of Rising Sun ; it is here 
found in scattered boulders and also exposed in the railroad cut 
near by. 
The gabbro-dioryte may be distinguished from the true 
dioryte both by its finer grain and by its dark greenish color. 
The latter characteristic also serves to separate it from the 
gabbro and noryte which do not have the greenish tinge impart- 
ed by the secondary hornblende when present. The metagab- 
bro is commonh' more compact than many of the pyroxene 
rocks. 
In the hand specimen the unaided eye can readily distinguish 
a clear, usually fresh, colorless or white feldspar and a green- 
ish, fibrous hornblende with a satiny lustre. The microscope 
shows that some magnetite, apatite and epidote are also pres- 
ent. The feldspar is bytownite as in the original gabbro and 
does not differ from that already described in connection with 
that type. It has not suffered much alteration and is as a rule 
clear and fresh ; occasionally, however, a little secondary ep- 
idote is developed in the plagioclase. The hornblende is the 
common green pleochroic variety. It never shows crystal out- 
lines but occurs in fibrous masses or forms aggregates com- 
posed of numerous small individuals. When the hornblende is 
fibrous, as is commonly the case, these individuals are often 
colorless on the inside and surrounded by a green and more 
compact border. 
♦The term gabbro-dioryte was first employed by A. K. Tornebohm (N'eues 
Jahrbuch f. Min. etc., 1877, p. 301), was later applied by Williams to tlie 
secondary diorytes of the Baltimore rcsion, and has since come into quite 
general use. Recently, however, it has been suggested that for much altered 
rocks which are of known igneous origin the term "meta" be ))refixed to the 
name of the original rook. In accordance with this usage an altered gabbro 
would be designated a metagabbro instead of a gabbro-dioryte. (Report of 
Committee on Nomenclature of Igneous Rocks, appointed by the Director of 
the United States Geological Survey.) 
