188 MINERALOGY AND LITHOLOGY. 
appear, no cavities containing fluid are found. It is considered that 
probably water played its important part in the crystallization of these 
rocks; but feldspar, even if it does imprison some water, does not often 
hold it, but allows it to escape through minute cleavages, or absorbs it 
into its decomposition products, while quartz holds what it contains 
through all time, and hence, even in the oldest rocks, the quartz is full of 
fluidal enclosures. 
In dismissing this subject it will be well to mention two or three less 
typical varieties which indicate how narrow are the spaces which divide 
our great families of rocks. On Little Deer brook, in Albany, a quartz 
porphyry occurs, in the ground mass of which quite large grains of mus- 
covite mica are developed. A specimen from Kirby, Vt., possesses but 
little ground mass, and what there is is coarse, and in it both muscovite 
and biotite are seen. A specimen from Newcastle is:also largely crys- 
talline, but hornblende, and not mica, is conspicuous in the thin sections. 
These circumstances indicate that between typical porphyries and typical 
granites, many intermediate varieties occur, which, from the nature of 
the case, are to be expected. 
Granitic Rocks. 
Under this head are included those massive acidic rocks which are 
entirely granular in their structure. They possess at times a pseudo-por- 
phyritic structure, induced by the more prominent development of ortho- 
clase crystals ; but to the unaided eye, all parts of the rock are plainly 
granular. They have certain features in common, which may be men- 
tioned before describing individual species. They all possess indications 
of having once been in a molten or plastic condition; and, like the rocks 
last described, the evidence they offer is not that of an igneous fusion, 
but rather of a plasticity induced at a low temperature, and under press- 
ure, by the aid of water. Therefore it may be again remarked in this 
connection, that we may expect to find these rocks in the places where 
they originally existed as stratified sediments, in dykes intruding them- 
selves through other strata, and in the most varied, apparently systemless 
forms which might happen to be imposed upon them by the circum- 
stances that befell them while yet plastic; and all these differences, 
