bascom.] BASIC ERUPTIVES. 69 
basalt (dolerite) as one of the degree and granosity of the crystal- 
lization. It is easy to see that these features are determined by the 
geological conditions of consolidation. It follows from this that the 
essential characteristics of the rock groups are independent of age. 
A history of the classification of the gabbro and its allied groups 
and an exact statement of the final definition of these groups have been 
concisely given by Dr. Bayley. 1 He suggests that the group of mela- 
phyres and augite porphyrites will eventually be dispensed with, when 
the olivine diabases and the diabase "will take the position thus left 
vacant, and the plagioclase augite rocks will be found to occupy these 
places with respect to each other; the gabbros, the position of a deep- 
seated rock; the diabases, that of the corresponding noncrystalline 
effusive; and the basalt, that of the hypocrystalline equivalent." 
With this understanding of the use of the terms diabase and basalt, 
the South Mountain basic rocks fall into the diabase group. 
They are noncrystalline, effusive, plagioclase augite rocks, with or 
without olivine. They' thus possess the characteristics of the augite- 
porphyrites and melaphyres (diabase group). Tiiey are so fine-grained 
as to appear homogeneous in the hand specimen, yet show no evidence 
in the thin section of an originally hypocrystalline character. There is 
no proof for or against devitrification. In the absence of such proof 
their present noncrystalline character will be recognized in their nomen- 
clature as primary. 
MELAPHYRES AND AUGITE-PORPHYRITES. 
DISTRIBUTIOX. 
The quartz-porphyries and aporhyolites in the Monterey district are 
limited to numerous small detached areas. The melaphyres and augite- 
porphyrites, on the other hand, occupy a large, irregular area, covering 
the valleys, the foothills, and the mountain flanks. Besides this area, 
which constitutes about one-half of the entire district, there are two 
small areas north of the old Maria Furnace, which are surrounded by 
the acid rocks, thus reversing the usual relation of the basic and acid 
eruptives. Along the State line and to the south of that line the dia- 
bases are massive or schistose, and inconspicuously amygdaloidal. In 
the Monterey district the amygdaloidal character of the diabases is their 
most marked feature. In the exposures on the Gettysburg Railroad 
narrow zones of inconspicuously amygdaloidal or nonamygdaloidal mel- 
aphyres grade above and below into consx)icuously scoriaceous rocks. 
Basic igneous slates occur at the west end of the Gettysburg tunnel, 
where they grade into massive diabases. They also occur on Colonel 
Benchoff's place, at a locality just north of Gum Spring, on a line 
northeast of the Blue Ilidge Summit station, and form a knoll south of 
the Fountaindale post-office. (See map, PI. I.) 
'The basic massive rocks of the Lake Superior region: Jour, of Geology, July- August, 1893, Vol. 
I, No. 5, pp. 433-456. 
