108 
AMERICAN GEOLOGY. 
Fig. 21 represents the rude columnar structure of the Pali¬ 
sades, which repose upon the lower members of the trias. The 
molten mass was also injected between the strata of sandstone. 
Greenstone is sometimes vesicular, but the vesicles having been 
filled with foreign substances, and remaining in relief after the 
rock is partially disintegrated, this variety is often called amyg¬ 
daloid, those foreign matters appearing like almonds, in the 
rock. Both greenstone and amygdaloid have a granular struc¬ 
ture, and feldspar may often be distinguished in small particles, 
disseminated through the mass. Greenstone is also porphyritic, 
the crystals of feldspar are distinctly formed during the pro¬ 
cess of cooling. 
§ 79. Basalt is a black compact rock, occupying the same 
relations to other rocks as greenstone, just described. It is both 
massive and columnar. Structurally it differs from greenstone, 
in its perfect homogenity. Frequently our trap dykes are per¬ 
fectly homogeneous and compact, and hence are basalts. Basalt 
is not a common rock upon the Atlantic slope. The rocks 
which have frequently been called basalt* are really greenstone, 
as either by the unassisted eye or by means of a single lens, 
particles of feldspar are visible. 
§ 80. Porphyry is a rock in which crystals of feldspar are 
tolerably well defined. They are embedded in a compact paste. 
This paste is often reddish or greenish, but the color is variable^ 
or it may be any color. 
Fig. 22 represents a 
porphyritic mass, which 
is common over a large 
passing southwesterly 
through Granville and 
Chatham counties. The 
porphyritic structure is 
rarely perfect. The masses of feldspar in the paste, though 
tolerably well defined, have rarely straight edges or even planes. 
area, in North Carolina 
particularly, in a belt 
Fig. 22. 
