C. IT. Smyth, Jr. — Gabbros in the Adirondack Region. 61 



I. II. in. 



Si0 2 46-85 44-10 46-85 



Al 2 O s - 18-00 24-86 19-72 



F 2 O s 6-16 7'89 3-22 



FeO 8-76 6-53 7-99 



MgO __ 8-43 3-89 7-75 



CaO 10-17 11-90 13-10 



Na„0 2-19 1-66 1-56 



K„0 0-09 0-24 009 



H~0 0-30 0-60 0-56 



Totals... 100-95 101-67 100-84 



Sp.gr 3-097 3-044 



The gabbro of the larger area at Morehouseville, as already 

 stated, gives only slight indications of its intrusive character. 

 The rock is decidedly coarser grained than that of the small 

 patches, and on the weathered surfaces closely resembles the 

 ordinary hornblende gneiss of the region. In thin sections it 

 shows the same constituents as do the finer gabbros, with the 

 addition of considerable apatite, while biotite is quite abundant 

 throughout the entire mass. The percentage of ferro-mag- 

 nesian minerals is smaller than in the other rocks, and the 

 augite is decidedly more abundant than hypersthene making 

 the rock a hypersthene gabbro rather than a norite. In struc- 

 ture it varies from completely massive to slightly gneissoid. 

 No crushing is apparent in sections, the undulatory extinction 

 is less marked than in the finer rocks, and there is no sign of 

 the stringing out of the hypersthene. 



It seems probable that the facts above stated may prove of 

 value in working out the geology of this region by giving a 

 clue to the origin of the black hornblende gneiss previously 

 referred to. Yery little has been done as yet along this line, 

 but a brief outline of the more important observations thus far 

 made may well be recorded in this connection, leaving the 

 details for a later paper. The black gneiss was first seen on 

 the shore of Big Rock Lake, about a mile and a half northeast 

 of Wilmurt, where it lies between acid gneiss below and 

 brown hornblende gneiss above. As the shores of the lake are 

 thickly wooded it is impossible to make out the character of 

 the contacts between the rocks. From this point the black 

 gneiss extends nearly or quite continuously along the strike, 

 till the head of Metcalf lake is reached, a distance of three or 

 four miles. Beyond this point the country has not been ex- 

 amined. On the north shore of Metcalf lake there is a series 

 of cliffs rising one above another in which the black gneiss is 

 seen alternating with layers of brown gneiss, sometimes with a 

 rather gradual transition between the two, but often with a 



