126 



NEW YORK STATE MUSEUM 



The critical points in the above discussion can be summed up as 

 follows : the criteria for the sedimentary origin of the amphibolites, 

 the presence of original quartz and motley collection of feldspars ; 

 for igneous origin, high pyroxene content and evenly " split " 

 feldspars. 



These criteria have been used in classifying the amphibolites 

 whose origin was not forthcoming from the field relations. How 

 successfully it has been done can not be tested at the present time, 

 but the hope is entertained that some progress has been made in 

 this difficult problem. The probable age relations of this rock have 

 already been touched upon. 



The Laurentian granite. The existence of a granite much older 

 than the Algoman series of eruptives in the Adirondack region 

 seems to the writer to have been sufficiently proved to need but 

 little comment. Its universal habit is to be intricately involved with 

 the Grenville series. This led the early geologists to regard it as a 

 Grenville sediment. 1 Even today this view is entertained by a few. 2 

 " The recognition of pegmatitic phases of the rock threw the first 

 doubt upon its sedimentary character," while " the chemical analysis 

 finally settled the question." 3 The writer wishes to add this addi- 

 tional bit of evidence which supports the contention that the rock 

 is of igneous origin. At the Dixon-Faxon and Hague localities it 

 was found that the lower beds of the Hague gneiss were soaked 

 and " smothered " by this rock, while the Hooper and Rowland 

 districts show that the Hague gneiss rests directly upon the Dresden 

 amphibolite, the granite being wanting. Its behavior in affecting one 

 stratigraphic unit here, and a different one there and its entire 

 absence in a third locality, is very suggestive of the igneous nature 

 of the rock. As to its age, the reader is referred to the Hooper and 

 Flake occurrences and to Cushing's reasons for regarding it as 

 Laurentian. 4 



SYNTECTIC ROCKS 



The lack of uniformity of the rocks, even those usually regarded 

 as wholly igneous, has led the writer to believe that many of the 

 rocks are of " composite character as a result of injection or 

 assimilation, giving on one hand a sediment more or less ' soaked ' 



1 Cushing, H. P., N. Y. State Mus. Bui. 77, p. 17-19; Kemp & Hill, N. Y. 

 State Geol. 19th Ann. Rep't, p. r 32-r 35. 



2 Miller, W. J., N. Y. State Mus. Bui. 182, p. 11. 



3 Cushing, H. P., N. Y. State Mus. Bui. 169, p. 21. 



4 Cushing; H. P., " Age of the Igneous Rocks of the Adirondack Region," 

 Am. Jour. Sci., 1915, 39:288-94, especially p. 292-93. 



