36 NEW YORK STATE MUSEUM 



pyroxene, plagioclase, orthoclase, quartz, garnet, magnetite, apaiite 

 and pyrrhotite. Two analyses were given showing marked contrasts 

 of composition. 



An exposure of what appears to be the same peculiar rock is to be 

 seen in the Mount Marcy quadrangle in the bed of Johns brook just 

 below its junction with Ore Bed brook and Slide Mountain brook. 

 A second series of exposures appears in the cascading portion of 

 another brook which comes down to Johns brook from the north- 

 west side of Wolf Jaws mountain. The writer's early observations 

 have again in this instance been corroborated and amplified by H. L. 

 Ailing. 



In the field one would be inclined to regard the rock as a member 

 of the basic gabbro series, but upon examination with the micro- 

 scope it is found to be very different both in mineralogy and texture. 

 The rock consists of irregularly shaped crystals of variable sizes 

 and in places apparently granulated from crushing. The chief feld- 

 spar is orthoclase, at times microperthitic. Carlsbad twins may be 

 detected. The microperthite is very fine, much more so than in the 

 general run of Adirondack rocks. There is some plagioclase, but 

 the extinction angles generally seem too small for labradorite, and 

 indicate a more acidic variety. In at least one slide quartz is in 

 notable amount. The dark silicate is emerald green augite, some- 

 times in relatively small anhedra, that is, 0.1-0.2 mm, sometimes in 

 large ones, 0.5-3.0 mm. Strong pleochroism, green to pale yellow, 

 may be obtained in favorable sections. The augite is in broken frag- 

 ments of most irregular outline. There are a very few shreds of 

 hornblende and biotite. The rock is richly provided with pink gar- 

 nets, which in small and large anhedra are disseminated through and 

 among the other minerals. They are at times in elongated shapes in 

 plagioclase apparently developed from certain favorable lamellae. 

 In amount the garnets rank well up with the feldspar and augite. 

 In one slide titanite in unusually large irregular masses, i. 0-2.0 mm, 

 is very prominent. Ilmenite altering to leucoxene, together with 

 apatite, and rarely pyrrhotite concludes the list of components. 



Apparently this rock can be satisfactorily explained only as an 

 old, surviving mass of Grenville gneiss, which became involved in 

 the intrusive anorthosites and affected with more or less of the 

 anorthosite substance. It shows the characters of both rocks. The 

 result has been a dark rock, resembling to the eye the basic gabbros, 

 containing at times large blue labradorite crystals, yet not agreeing 

 with the basic gabbros when studied with the microscope. This 



