LITHOLOGY. 185 
resting quietly in their original béds. If the eruptions reached the sur- 
face of the earth the pressures would be relieved; and as the cavities 
point as before to very heavy pressures, we are led to suppose that such 
disturbances are the result of movements which took place in the earth’s 
depths rather than upon its surface. The circumstance of the elevation 
of our mountains is sufficient to make plain the possibilities of such sub- 
terranean movements. This is an interesting spot, for such a mixture 
of stratified and unstratified material is not often seen. 
Quite a variety of quartz porphyries are found in this region. Some 
are colored red by the oxide of iron that seems to have separated from 
the decomposing minerals; in some, but few crystals are porphyritically 
developed, and some others are nearly granitic in their texture. On the 
lower half of the mountain a gray porphyry is prominent, which possesses 
a very fine, compact, rather earthy-appearing ground mass, in which not 
only feldspar crystals are developed, but also numerous dihexahedrons of 
quartz. Such porphyries as this were once called clay-stone porphyries, 
on account of the appearance of the ground mass. Another specimen 
of this rock has a basis still more earthy in appearance, which results 
from the abundant presence in it of chlorite, biotite, and iron oxide, 
which are only microscopically identified. Moat mountain, and some 
other mountains surrounding Pequawket, are largely composed of por- 
phyries. 
A porphyry just like this, save that in it very much larger crystals of 
orthoclase and quartz are developed, occurs in the Notch in the White 
Mountains. This porphyry is considered by Prof. Dana to form a bed, 
the position of which is conformable with the strata of slates and schists, 
and thus it presents a case of a porphyry which was not disturbed from 
the place where its metamorphism took place. 
A most diversified collection of porphyries can be obtained at Water- 
ville. They are of various colors, owing to combinations of differently 
colored ground mass and crystals, but the prevailing color is red. This 
color is seen in thin sections to be due to oxide of iron, which is abun- 
dant in the ground mass, and is often spread through the minute fissures 
in the orthoclase crystals. Sometimes a black grain of iron oxide in a 
strong light will appear blood-red, and it is thus recognized as hematite. 
These porphyries also vary widely in composition. In some, quartz is 
VOL. Iv. 24 
