Some Metamorphosed Eruptives of Maryland. 
159 
color makes it apparent in contrast with the green of the surrounding rock. 
The feldspar of this rock is a basic plagioclase, corresponding closely with 
bytownite. Between crossed Nicols its strained condition is apparent in 
the wavy extinction. The hypersthene is strongly pleochroic, and, like 
the diallage, has the usual characters except for the marginal rim of horn¬ 
blende. 
The gabbro diorite contains neither hypersthene nor diallage, both hav¬ 
ing been changed completely to hornblende. This mineral is not generally 
so fibrous as that of uralitized diabases, but it is a green massive variety 
in crystals not many times smaller than those of the pyroxene from which 
they are derived. Not infrequently the central portions contain areas, 
roughly circular, of a colorless mineral of low refractive index, which is 
believed to be quartz separated in the alteration process, and indicating 
that its host is less acid than the original pyroxene. Similar areas occur 
in the hyperite-diorites of Sweden and in the Baltimore gabbro-diorite. 
The feldspar is bytownite, but here it shows more marked optical ano¬ 
malies than in the gabbro.*?' In the more effected specimens, hornblende 
is developed in needles within the feldspar, being sometimes arranged in 
bands and indicating, it would seem, lines of weakness. In certain of these 
specimens a distinct reaction-rim appears between the feldspar and the 
hornblende. Between crossed Nicols this rim affords high colors like those 
of epidote. Epidote was found by Dr. Williams to compose reaction-rims 
in the gabbro-diorite of Mount Hope, and it was at first suspected that 
these aggregates might be composed largely of epidote. Prof. Rosenbusch 
suggested to me that the mineral is amphibole in thin scales, the high col¬ 
ors resulting from intercalated films of air. (Fig. 1.) 
In some specimens of this rock 
rutile is present as an important ac¬ 
cessory constituent, associated with 
ilmenite, and both are surrounded 
by wide rims of titanomorphite or 
sphene. (See Fig. 2.) In a few local- 
ties (Ilchester R. R. cut) these min¬ 
erals can be seen in the hand speci¬ 
men, where they closely resemble 
the similar association in the horn¬ 
blende gneiss of Lampersdorf in 
Silicia. As soon as the rock takes 
on the laminated character of a 
gneiss, the feldspars appear broken 
down or granulated peripherically 
into fine mosaics (Randliche Kata- 
Fig. 2 .— Section of gabbro-diorite from 
Bchester, Md., showing ilmenite, rutile and 
titanomorphite. X275. 
klassstruktur.) Simultaneously a “fraying out” and splintering of 
hornblende, and a considerable development of epidote are to be 
