GEOLOGY OF THE NORTH CREEK QUADRANGLE 3 1 



markable manner in the North Creek region gabbros. In the ex- 

 amples most often described the core is oHvine, but in the gabbros 

 here considered the writer has observed cores of oHvine, hypers- 

 thene, ilmenite, augite, and diallage with from one to five distinct, 

 successive rims surrounding the cores. Professor Kemp has de- 

 scribed ^ and figured a number of interesting examples of reaction 

 rims observed in certain gabbros of the eastern Adirondacks. 



The following nine types of reaction rims comprise most of those 

 noted by the writer in the North Creek gabbros : 



1 Ilmenite surrounded by hornblende. 



2 Diallage surrounded by hornblende. 



3 Augite surrounded by hornblende. 



4 Hypersthene surrounded by garnet. 



5 Hypersthene surrounded by successive zones of biotite and 

 hornblende. 



6 Olivine with successive zones of hypersthene, hornblende, and 

 garnet. 



7 Olivine with successive zones of hypersthene, biotite, and garnet. 



8 Hypersthene with successive zones of biotite, feldspar, and 

 garnet. 



9 Ilmenite with successive zones of biotite, hornblende, garnet, 

 and biotite. 



In nearly all cases the material immediately inclosing llie rims 

 is feldspar which, in a sense, adds another zone to each of the abo\ e. 

 No. 6 is like one of those described by Kemp. No. 9 is a remark- 

 able example and, because of its additional outer rim of biotite, is 

 even more interesting than a case described by Lacroix.^ Some of 

 the others may be new examples. The material of each rim appears 

 to be highly granulated or at least made up of numerous small grains. 

 It seems certain that where hypersthene envelops olivine, the former 

 has secondarily developed from the latter. The olivine cores are of 

 very irregular shapes and in all sizes. Where hypersthene forms 

 the core it is probable that all the original olivine has been altered 

 to hypersthene. The common occurrence of hornblende rims around 

 pyroxene strongly suggests the derivation of the former from the 

 latter. Garnet is almost invariably in contact with feldspar which 

 suggests the partial formation, at least, of the- garnet from the 

 feldspar. 



^ Geo!. Soc. Amer. Bui., 1894, 5:218-21. 

 2 Bui. Soc. Min. Fr., 1889, 12:232. 



