76 VOLCANIC ROCKS OF SOUTH MOUNTAIN. [bull. 136. 
product. Feldspar is often fresh and unaltered and the structure 
preserved. 
In the first three types actinolite may also be present, but not so 
abundantly as in the last. The first type passes into the second by 
increase in epidote, and the second type grades readily into the third 
by increase in chlorite. As this increase accompanies the development 
of schistosity, most of the schists belong- in the third group. 
Types 3 and 4 are not sharply separated. Chlorite and actinolite are 
present in both. In the former chlorite predominates, and in the latter 
actinolite. 
Olivine may be present in any of the four groups. The crystal out- 
lines are best preserved in the first group; hence the ophitic structure 
is here best preserved; and it is most obscured in group 3, where orig- 
inal crystal outlines are lost. 
Group 1 contains the rocks which have been the least sheared, a fact 
which is perhaps accounted for by the silicified character of the rock. 
The peculiar dike-like band which traverses the basic eruptives at 
the west end of the tunnel is not unlike type 1 in color and compactness 
of texture. The color is several shades darker, and the specific gravity 
of the rock is greater. Under the microscope a further remsemblance 
is seen. The compact rock mass consists largely of titaniferous mag- 
netite (arranged in layers, or outlining obscurely an ophitic structure), 
chlorite, epidote, and quartz. 
The amygdaloidal selvage of the band shows the same constituents 
in an inverse proportion, and the ophitic structure is strongly marked. 
This was true in all the numerous thin sections made of the baud. In 
the vesicular portion of the band the ophitic structure is well pre- 
served, and even olivine crystals with unaltered outline are present. 
The amygdules are filled with quartz, granular and crystalline epidote, 
and chlorite. It is probable that the yielding of the vesicles, which 
would offer the least resistance to pressure, saved the rock. The 
tendency to a parallel arrangement of the feldspars on either side of 
the amygdules accords with this supposition. The passage of this 
baud, which is sometimes only from an inch to two inches wide, into a 
green chlorite-schist or slate is very abrupt. The difference seems to be 
due to the presence in the band of a large amount of iron. Its chem- 
ical analysis, given on page 78, coincides with this view. 
Epidote and quartz are by far the most abundant and widely dis- 
tributed of the secondary minerals. This is true not only of the basic 
eruptives, but also of the acid eruptives Avhen they are the prevalent 
alteration products and amygdaloidal filling, and where piedmontite, 
a member of the epidote group, occurs in macroscopic quantities. 
There seem to have been conditions favoring an extensive epidotization 
and silicification. Undoubtedly there has been within the rocks them- 
selves a mutual reaction between the decomposition products of feld- 
spar and augite resulting in the production of epidote. The fact that 
