110 NEW YORK STAT® MUSEUM 



rocks. The border parts of the bosses are quite similar, but 

 centrally they are much coarser, are more feldspathic, are of gra;y 

 color spotted with black, and their characteristic ophitic structure* 

 and corrosion rims are at once evident to the eye. 



As a result of the metamorphism which they have undergone,, 

 these gabbros are in large part converted into amphibolite 

 gneisses. Most of the bosses show a comparatively unchanged 

 core, from which a regular gradation in structure and mineral 

 content into the amphibolite can be traced. In some the conver- 

 sion is almost complete, but these usually furnish material front' 

 their central portions which exhibits traces of the original miner- 

 als and structure in thin section. Still others show no such traces, 

 and these last are, with our present knowledge, absolutely not 

 to be distinguished from other amphibolite gneisses which have 

 been provisionally classed with the doubtful gneisses and already 

 described. Quite similar rocks occur also in close association 

 with the crystalline limestones and graphitic gneisses of the 

 Grenville series. The writer regards them all as metamorphosed 

 igneous rocks, either gabbro or some related rock, but believes 

 these last to be much older than the gafobros immediately under 

 discussion. Certain small areas of amphibolite are met with r 

 however, which can not be classified with certainty with either. 



Much the same phenomena are presented by the dikes. These 

 show considerable variation in thickness but are mostly under 30 

 feet and often only a foot or two. Some of them have their orig- 

 inal character in large part preserved, others are wholly changed 

 into amphibolite, yet others show intermediate stages. The- 

 wholly changed dikes are so like other schistose, amphibolite 

 dikes which occur all over the Adirondack region that the writer 

 is not certain that they are not the same, though the ordinary 

 rusty appearance of the latter, coupled with the fact that they 

 have not been noted cutting any of the certain later eruptives 

 (anorthosites and syenites) disposes him to regard them as of 

 different and earlier age. Their mineralogy is precisely the same r 

 except that garnet is not so abundant as it usually is in the other 

 rock. 



