r58 NEW YORK STATE MUSEUM 



5 that the three together, anorthosite, syeniite and granite, form 

 a great eruptive complex in the heart of the Adirondack region^ 

 and that all are younger than the (in parft at least) sedimentary 

 Grenville rocks. 



The writer was at first disposed to take a different view from 

 that given above, thinking that the probable explanation of the 

 phenomena was the presence of augite syenite of two different 

 ages in the region, the one younger and the other older than the 

 anorthosite, the last to be classed in the general " gneiss " series 

 of the Adirondacks. While that may yet prove the true expla- 

 nation, the probabilities seem to favor the one outlined above,, and 

 some discussion of the subject seems desirable, since the deter- 

 mination of the truth or falsity of either view is a matter of much 

 importance in Adirondack geology. 



Classification of the Adirondack pre-Cambrian 



The classification of the pre-Cambrian rocks which we have been 

 tentatively using in the Adirondacks is one proposed some 10 

 years ago by Kemp. Excluding certain dikes, the rocks were 

 divided into three groups. 



Series 1 was defined as a great group of gneisses of diverse 

 kinds and of somewhat uncertain origin, though likely in great 

 part igneous. 



Series 2 comprised the crystalline limestones and associated 

 quartzose, sillimanite and graphitic gneisses, all representing 

 metamorphoised elastics, and closely interbanded with other, 

 notably basic, gneisses which are probably igneous but can not 

 be separated from the main group. 



Lest it should be imagined that this order carried any implica- 

 tion as to the relative ages of the two groups, it should be stated 

 that the grouping is a wholly lithologic one, and that Kemp has 

 constantly maintained that there was no evidence that any of the 

 rocks included in series 1 were older than the rocks in series 2^ 

 though admitting that older rocks might be represented.^ 



^ Kemp, J. F. N. Y. state geol. 13th an. rep't. 1893. 1 : 448- 



