Smyth — Crystalline Books of St. Lawrence County. 



has Iteen crystallization after the production of cataclastic structure in the 

 nick. While in many cases the quartz was probably an original constituent 

 and has recrystallized ; in others, it is doubtless secondary, the gneiss being 

 more acid than the parent rock. The microscopic details upon which these 

 conclusions are based need not be state* 1 here. 



Where the cataclastic structure is lacking, the gneisses have the non- 

 crystalline granular structure of plutonic rocks, the grain varying con- 

 siderably, and, in the coarse varieties, porphyritic structure being particularly 

 common. Rapid variations of grain are often shown by the coarse porphy- 

 ritic varieties, fine phases coming in quite irregularly, forming bands and 

 masses with the aspect of the common, tine gneiss. They lack, however, 

 the crushed aspect, and appear to be primary variations. The micro- 

 scopic study necessary to determine this point has not yet been under- 

 taken. The same localities often show the passage from the massive 

 into the finely laminated crushed gneisses, the transition sometimes 

 occurring within a few feet. It often happens that the passage from a 

 coarse, massive, to a fine, w ell laminated gneiss is attended by a darkening 

 of the color. 



But in other cases, where the coarse, light-colored gneiss occurs with a 

 tine dark-colored variety, there is good reason for believing that a different 

 relation exists betw een the two. In instances of the latter kind, the fine gneiss 

 is usually cut by abundant granitic veins, which wander irregularly through 

 the rock, with a tendency to follow the foliation if it is at all marked. At the 

 same time, the acid gneiss contains masses of dark rock of great variety 

 of form and size, and, in everv respect like inclusions in an igneous rock. 

 That such they really are scarcely admits of a doubt, and all uncertainty is 

 removed where, as has happened in several instances, an irruptive contact is 

 found between the light and the dark gneiss. Such phenomena point to two 

 conclusions: first, all parts of the gneiss are not of the same age; and 

 secondly, the massive, porphyritic gneisses are certainly in part, probably 

 entirely, of igneous origin, being younger than, and intrusive in the fine, dark, 

 gneisses. These facts, however, shed no light upon the origin of the latter 

 variety. 



The inference above drawn, as to the true nature of the porphyritic 

 gneisses, becomes important in its hearing upon the general question in hand, 

 when it is remembered that these rocks themselves make up a not inconsider- 

 able part of the gneissic areas, and, moreover, that much of the finer gneiss 

 is probably derived from them. 



