bascom.] MELAPHYltES AND AUGITE-PORPHYRITES. 75 
little quartz. In this case epidote is the predominating alteration 
product These rocks are recognized in the hand specimen by a light- 
green color and green amygdules. Feldspar, augite, and olivine have 
all been replaced by epidote. The material for this mineral has 
undoubtedly been furnished by the interaction of feldspar and augite, 
and also has been brought to the rock by percolating water from over- 
lying rocks. In the extreme phase of this alteration these rocks are 
trueepidosites. 
In the presence of a larger amount of iron, actinolite is abundantly 
developed. There is also much free iron oxide, and the rock becomes 
dark green in color. Where there has been shearing movement, as in 
the case of the acid rocks, a micaceous mineral is developed. In the 
aporhyolites the mineral is sericite; in the porphyrites it is chlorite. 
In the incipient stages of the schistose structure chlorite occupies the 
center of the amygdules, with quartz and epidote filling the rest of the 
space. When the rock, is so schistose as to be fairly called a slate the 
amygdules are represented by brilliant dark-green spots and consist of 
chlorite only. 
Chlorite in turn becomes the prevaling alteration product. Often 
none of the original constituents remain. Actinolite, chlorite, epidote, 
and secondary silica are the invariable constituents of the "spotted 
greenstone schists." The actinolite and chlorite both blur the outlines 
of the original constituents and obliterate the original structure. These 
rocks are a medium green in color. 
The important secondary constituents of the porphyrites are quartz, 
epidote, actinolite, chlorite, and leucoxeue. The prevalence of one or 
the other of these alteration products can be determined in the hand 
specimen by means of the color of the rock and the character of the 
amygdules. With reference to the character of the alteration which 
they have undergone, the melaphyres and spilites thus fall into the fol- 
lowing groups: 
1. A blue-gray rock with quartz amygdules which do not show 
shearing. Under the microscope it shows an ophitic structure well 
preserved, and a silicified groundniass. Localities: Near Gum Spring, 
on the Old Furnace road, on Minie Branch, and along the Gettysburg 
Railroad . 
2. A light yellowish-green rock with epidote-quartz amygdules, and 
epidote as a prevailing constituent. The original structure is obscured. 
Localities: South of the Clermont House on the Gettysburg Railroad 
and at the Russel copper mine. 
3. A medium-green spotted schist. Chlorite is the prevailing min- 
eral. The original structure is more or less completely obscured. 
Localities: Along the State line at the west end of the tunnel and at 
many other places. This is a prevailing type of the porphyrite. 
4. A dark-green rock, more or less schistose. Epidote, quartz, and 
chlorite form the amygdules. Actinolite is abundant as an alteration 
