T. S. Hunt on Lithology. 257 
the principle which I have laid down of the comparative rarity 
of exotic rocks in crystalline and metamorphic regions. 
Oceasionally the crystallization of a rock takes place around 
certain centres, giving rise to rounded masses which have a ra- 
diated or a concentric structure, and constitute the so-called glob- 
ular or orbicular rocks. Distinct crystals of some minerals, gen- 
erally feldspar, augite, or olivine, are often found im ed in 
rocks having a compact base. To such rocks the name of por- 
phyry is given, and by analogy a rock with a granular base en- 
closing distinct crystals is designated as porphyritic or porphy- 
roid. Amorphous or vitreous rocks, as pitchstones, are in like 
manner sometimes porphyritic. The name of porphyry, at first 
given toa peculiar type of feldspathic rocks, has now become so 
extended that it is to be regarded as only indicating an accident 
of structure. The title of amygdaloid is given to various rocks 
having rounded cavities which are wholly or partially filled with 
various crystalline minerals. The base of these rocks is gener- 
y granular or crypto-crystalline, but is sometimes amorphous, 
resembling a scoria or vesicular lava, the cavities of which have 
been filled by infiltration. Such is doubtless the origin of some 
amygdaloids. In more cases however these cavities have prob- 
ably been formed like those often found in dolomites and some 
other rocks, by a contraction during solidification. Porphyroid 
rocks, in which quartz, orthoclase and other minerals are ar- 
Tanged in orbicular masses, are also sometimes designated amyg- 
daloids, and may be confounded with the two previous classes m 
which thé imbedded minerals are the result of subsequent infiltra- 
tion. Allied in structure and origin to the last are what are named 
Variolites or variolitic rocks. (See Geology of Canada, pp. 606, 607.) 
The masses into which some aluminous minerals enter as a 
classes, whose origin we mane“ aneae 
dy referred to. (This Journal, [2], xxxvi, 218.) ‘The first of 
Mant mineral is orthoclase, generally associated with quartz, and 
the composite rocks of this class seldom have a density much 
above that of these species; or from 2°6 to 2°7. In the second 
Class, the characteristic mineral is a triclinic feldspar, with pyrox- 
