C. H. Smyth, Jr. — GaUbros in the Adirondack Region. 63 



bands seen around this mineral in hand specimens. The 

 tongues are usually curved and when developed from several 

 centers give a remarkably beautiful effect under the micro- 

 scope, impossible to describe or to figure with accuracy. An 

 attempt in the latter direction is shown in figure 4. This 



Fig. 3 — Gabbro. Tongues of hypersthene radiating from garnet. The feld- 

 spar somewhat altered. Diam. of field - 7 mm . 



Fig. 4 — Black gneiss. Tongues of hypersthene and hornblende radiating from 

 garnet. Feldspar completely altered. Diam. of field 2 mm . 



structure is analogous in every respect to that described in the 

 gneissoid gabbro, differing from it only in being on a larger 

 scale, with all of the constituents coarser, and in the presence 

 of tongues of hornblende as well as of hypersthene. The analogy 

 is rendered even more complete by the fact, not previously 

 stated, that in the sections of gneissoid gabbro which contain 

 grains of garnet the latter are partially surrounded by radiat- 

 ing tongues like those around the garnet of the gneiss, but often 

 so slender as to make difficult an accurate determination of the 

 mineral composing them. This structure is illustrated in 

 figure 3. 



Thus, the black gneiss is closely related to the gabbro in 

 mineralogical composition, showing such differences from it as 

 might result from alteration and metamorphism. At the same 

 time the gneiss has developed on a large scale a peculiar struc- 

 ture which is precisely analogous to a structure shown in the 

 gneissoid gabbro. From these facts it seems justifiable to con- 

 clude that at least there is great probability that the black 

 gneiss is a metamorphosed gabbro. The final establishment of 

 the truth or error of this hypothesis will be an important step 

 in unravelling the history of the gneissic complex of the re- 

 gion, for the bowlders of the drift indicate that the black 

 gneiss is a rock of considerable importance. In fact, it may 

 be said of all the rocks described that they are doubtless of 

 much greater extent than might be inferred from the facts 



