36 



XEW YORK STATE MUSEUM 



assume a deep-brown color. The larger and more typical stocks 

 commonly have a nonmetamorphosed interior facies which has a 

 more or less well-developed diabasic texture, and a border facies 

 (often amphibolite) which is so thoroughly gneissoid that the dia- 

 basic texture has disappeared. The typical nonmetamorphosed 

 gabbros, with diabasic texture and large percentage of black 

 minerals, is readily distinguished from all other rocks of the region. 

 Some of the border facies, however, are either amphibohtes which 

 look much like certain dark gneiss inclusions conmaonly seen in the 

 granite and syenite, or dark gneisses which might be easily con- 

 fused with certain basic facies of the syenite. 



Under the microscope the mineralogical composition of the gab- 

 bros is shown by the following examples taken from various stocks 

 or dikes. 



Table f — Thin sections of typical gabbros 











































































Sidsna. 



















































1 



1 



1 i 



'- 



- 



~;i 





a: 



^ 





3 





~ 





— 



— 



~ 



— 





"^ 



— 



- 



"^ 







-• - -•- 





















15 



.-^-l 



-.-1^:. : 





11 



17 





^ 



Ettle, 

 2;. 



Httfe 



25 . 

 15 . 





17 





25 



>: zi -^ 



.--n-l.:;. ; 







J 





■ji 



mad. 





I . 





3- 





Ar.-_,i:. 5 













? . 







little 







-o. ---.-_. : 



















wes:- 



7:. 7 l?.rre5: gabbro stock, 4 mi'r^ t -: :: 1 ir lake village, is 

 t: : : ' no. 52, table 6) ani '.::- v :t: :; with the 



: : t irder facies well ex: : 7 ;^ 



: :^T = : island in El. t 1^ 1 : 2J, 



: T : : :: 5: typical, ::f :: : :: :;_: r 7 : ^ t 55oid, 



It ::ed and with only slightly developed diabasic texture, 



T : rif- facies is finer grained, more granulated, and 



?.::/:l. : 1 T 7 .'ery similar rock (no. 25, table 6) outcrops on 

 the small island just to the southeast and it is probable that these 

 two masses are parts of the same gabbro stock now mostly covered 

 by water. If they are connected, the submerged portion may be 

 nonmetamorphosed and with better diabasic texture. 



