GEOLOGY OF THE NORTHERN ADIRONDACK REGION 



329 



reacting minerals and may perhaps have developed during the 

 cooling of the rock, since they are found in the least metamor- 

 phosed portions, and since such phenomena are known in other 

 gabbros which have not been metamorphosed. In this manner are 

 formed the well known " corrosion rims w which appear in many 

 gabbros in all parts of the world. 



In addition to these minerals there appear others in the granu- 

 lar condition in all rocks in which there is any trace of meta- 

 morphism. These are in the main the same as the original 

 minerals of the rock, plagioclase (mostly labradorite) , augite, 

 hypersthene, and green hornblende. It seems quite certain that 

 the material for the formation of these has been derived from the 

 original minerals of the rock, and that the process has been one 

 of granulation and recrystallization. The newly formed labra- 

 dorite and augite are entirely lacking in the multitudinous in- 

 clusions which are so characteristic of the original minerals, and 

 in virtue of which even the smallest remaining fragments of them 

 may be detected. The original minerals may often be seen tailing 

 off into granular material, which has evidently formed at their 

 expense. But the grains are not mere shattered fragments of the 

 larger crystals, but consist of a mixture of all the minerals men- 

 tioned above. 



Additional minerals which are usually or occasionally present 

 are apatite and titanite frequently, pyrite and pyrrhotite occasion- 

 ally, and sometimes a little green spinel (pleonaste). Scapolite 

 is sometimes present as an alteration product, and in the gneissoid 

 gabbros there is often considerable untwinned feldspar which may 

 be orthoclase. 1 



Some of the more important localities in the northern Adiron- 

 dacks where these rocks occur are as follows : 



In Clinton county at Keeseville. where occasional dikes of the 

 gabbro are found cutting the anorthosite gabbro, showing that 

 they are younger; at Petersburg, where exposures of a very wide 



x More detailed descriptions of some of these rocks may be found in the 

 following papers : 



Kemp, J. F. Am. Jour. Set August 1892, p.100-14. 



Geol. Soc. Am. Bui. 5 :213-24. 



Smyth, C. H. jr. Am. Jour. Sci. July 1S04, p.54-63. 



Am. Jour. Sci. April 1896. p.273-31. 



Geol. Soc. Am. Bui. 6 :268-83. 



