I 



GEOLOGY OF THE LAKE PLACID QUADRANGLE 59 



Gabbro and Metagabbro 



Seven gabbro masses are represented on the accompanying geo- 

 logic map. These are in most respects quite Hke the usual gabbro 

 of this age throughout the Adirondacks. A rather full account of 

 the typical gabbro is given in the writer's State Museum report on 

 ".The Geology of the North Creek Quadrangle." Most of the gab- 

 bro masses of the Lake Placid quadrangle appear to occur as true 

 stocks rather than as dikes. No tongues or branches from them 

 were observed to extend into the country rocks. The stocks all 

 have rounded or elliptical ground plans, and range in size from 

 one-eighth of a mile across to i mile across. Most of the gabbro 

 bodies are certainly intrusive into, and therefore, younger than, the 

 syenite-granite series, but, judging by experience in the Lyon Moun- 

 tain quadrangle some may be older than the syenite-granite. They 

 are clearly older than the diabase dikes. Whether they are older or 

 younger than the gabbro-diorite dikes above described could not be 

 positively determined, but they are probably younger. 



The gabbro bodies nearly all consist of nongneissoid interior 

 facies with more or less well-developed diabasic texture, and very 

 gneissoid (amphibolitic) border facies without diabasic texture. 

 The nongneissoid diabasic textured gabbro is generally easily dis- 

 tinguished from the other Adirondack rocks, but the amphibolitic 

 border facies often greatlv resemble certain of the Grenville horn- 

 blende gneisses. 



The fresh rock is dark gra}^, which, on weathering changes to 

 deep brown. Most of the rock is medium grained. About equal 

 amounts of plagioclase (chiefly labradorite) and dark minerals 

 make up the main bulk of the rock. Portions or all of the labra- 

 dorite crystals are often filled with tiny dark specks of some 

 unknown mineral arranged parallel to the twinning bands. Most 

 prominent of the dark minerals are monoclinic pyroxene, hyper- 

 sthene and hornblende. Garnet seldom fails, and it often consti- 

 tutes 10 or 15 per cent of the rock. The pyroxene and garnet are 

 commonly much granulated, the crushed garnet generally either 

 forming rims around feldspar or granulated pyroxene, or borders 

 between feldspar and granulated pyroxene. Olivine occurs in at 

 least two of the masses, this mineral being rather uncommon in 

 Adirondack gabbros. Slide 52 of table 5 shows olivine with suc- 

 cessive rims of granulated hypersthene, feldspar and garnet (see 

 lower right figure of plate 18). Table 5 gives the mineralogical 

 ctlinpositions of several typical gabbro bodies. 



