no American Quarterly Microscopical Journal. 



been to indicate the main facts of a field of investigation 

 rarely familiar to microscopists, as well as the particular views 

 of certain writers which do not seem to present a wholly satis- 

 factory explanation of some of the following facts. 



In the counties of New York and Westchester, in this state, 

 a somewhat micaceous and fine-grained, blackish-gray gneiss 

 occvirs in considerable abundance, which possesses no physical 

 characteristics of special importance, except the frequent con- 

 centration of quartzose aggregates of fibrolite, iron-garnet, and 

 black tourmaline in thin lenticular seams. I have given par- 

 ticular study thus far to the microscopic character of this gneiss 

 only in specimens collected from the vicinity of the town of 

 New Rochelle, in Westchester county. Its thin section here 

 reveals the following constituents : 



Quartz predominates in colorless and angular granules of 

 rather uniform size. Its clear material is sometimes traversed by 

 irregular fissures and generally slightly clouded by long, straight 

 and linear groups of inclusions (X 170); under higher power 

 (X500) these are clearly resolved, mostly into cavities of len- 

 ticular, angular, elliptical, and circular forms filled with liquid 

 gas, or liquid with a bubble ; the bubble in some groups is 

 stationary, in others exhibits a greater or less mobility, from a 

 tremulous vibration to a lively dance in every direction, many 

 liquid inclusions with bubbles in motion being visible at one 

 time. 



On focussing at greater depths, the inclusions are found to 

 lie mostly in planes, of which those of neighboring groups are 

 nearly parallel, with shorter branches either connecting neigh- 

 boring groups, or bifurcating, and often thinning out complete- 

 ly but indistinctly within a quartz field. There is the great- 

 est variety in the numbers and distribution of the cavities with- 

 in each plane. In size, they may, perhaps, average about 

 0.00014 mm., but they vary from minute specks, too small for 

 measurement, up to 0.0054 mm., or larger. Their forms show 

 a wide variation, as already described; but, although generally 

 rounded, most of the cavities present one, or more plane-faces, 

 a straight side in cross-section, or even a distinct and sharp 

 outline like that of a quartz-crystal, produced by negative or 

 inverted crystallization— a form often observed by Vogelsang 

 and others. Many of these forms, focussed at different depths, 

 are exhibited in the drawing of a single group (Fig. 2) 



