1 88 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 famihes 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, 



