72 VOLCANIC ROCKS OF SOUTH MOUNTAIN. [bull. 136. 
band are all more readily explained on the supposition that we are deal- 
ing with a genuine igneous dike than in any other way. The structure 
of the rock under the microscope and its analysis (see Analysis III, 
p. 78) show that there is no essential difference in these characters 
between the band and the schistose diabase which it traverses. Its 
chemical composition differs only in the high percentage of iron which 
it carries. It is possible that for some cause there has been a local con- 
centration of iron (to which the color is due) within the limits of this 
band, which renders it harder than the surrounding diabase and enables 
it to resist pressure more successfully. Hence, while not yielding itself 
to the pressure which produced the schistosity of the diabase, it has 
also been the means of producing a foliation in the diabase parallel to 
itself. The only other tenable hypothesis is that it represents a later 
intrusive lava flow of the same general composition as that of the rock 
into which" it was intruded. The manner in which it grades into a 
finely vesicular rock on its upper surface, and the inclusions of frag- 
ments of a green diabase, would be explained by this hypothesis. Its 
resistance to pressure would be due to the same cause in either case. 
In only a few instances do the diabases show a porphyritical struc- 
ture apparent to the naked eye. The diabases have not infreqently 
suffered crushing, and are recemented with quartz, epidote, and hema- 
tite, the former minerals predominating. Veins of asbestos with quartz 
occur in the more epidotic diabase. 
MICROSCOPICAL DESCRIPTION. 
Original structures. — There is a marked uniformity of structure and 
of mineral constituents in the South Mountain diabases. 
Unlike the aporhyolites, the porphyrites and melaphyres do not show 
the effects of magmatic movement. Their structure is universally the 
ophitic, which is produced only in a magma in a state of equilibrium. 
The vesicles also, as has already been noted, do not betray any fluidal 
movement. (PI. XXVIII, a.) 
Crystallization is fine-grained (see p. 69), corresponding to what has 
been called the u microophitic." That originally this microophitic struc- 
ture was associated with and passed insensibly into the hyalopilitic is 
not impossible, although subsequent processes of alteration, chief among 
which is silicification, have destroyed all trace of an unindividualized 
base. 
Shearing has obscured and sometimes obliterated the delicate ophitic 
structure through processes detailed later. Where dynamic action 
found relief in the crushing of the rock rather than in the production 
of a schist, the ophitic structure remains perfectly preserved in the rock 
fragments. The porphyritic structure is inconspicuous. Among the 
nonolivinitic porphyrites intratelluric crystallization is nearly absent. 
Feldspar and augite phenocrysts are rare. This characteristic, together 
with the widespread development of the amygdaloidal structure, allies 
these rocks to Bosenburch's spilite type. 
