502 NEW YORK STATE MUSEUM 



Summary and conclusions 



The investigation of the gneissic area resulted in showing the 

 possibiHty of splitting up the " doubtful gneiss " of earlier reports 

 into three types : ( i ) Syenite, which is igneous in origin and is in all 

 respects similar to the syenite previously described in other Adiron- 

 dack localities. (2) Granite. (3) Quartzose gneiss of sedimentary 

 origin, which may be the rock that has sometimes been termed 

 " gneiss of the limestone series." The syenite is without doubt a 

 plutonic igneous rock, and although gneissic phases are common, 

 completely massive ones predominate. The granite is more com- 

 pletely gneissic, and for that reason there is less certainty in deter- 

 mining it. Both syenite and granite are alike in presenting varia- 

 tions in the percentages of ferro-magnesian constituents. The third 

 type of gneiss is the most highly metamorphosed. It contains so 

 many intrusions of small size both of syenite and of granite that it 

 was found impossible to mark them off in mapping. It was further 

 altered by secondary infiltration of quartz, both in the form of large 

 veins and of disseminations of microscopic size. 



The presence of these small intrusions affords evidence that the 

 granite and syenite are younger than the quartzose gneiss ; and the 

 character of the rock, its macroscopic and its microscopic appear- 

 ance, and the topography of its mountains point toward a sedi- 

 mentary origin. Its frequent association with the limestone (occur- 

 ring sometimes in thin layers folded with the limestone, sometimes 

 in hills while the limestone occupies the intervening valleys) points 

 toward the conclusion that this quartzose gneiss is a member of the 

 limestone series. Since this gneiss underlies the limestone, and also 

 underlies the other gneisses which are of sedimentary origin, it is 

 thought to represent the base of the Grenville series. In its basal 

 position is to be found the explanation of the great number of 

 intrusive masses which render this rock so difficult of interpretation 

 in the field. Being at the bottom of the sediments, it formed the por- 

 tion most subject to alteration from the intrusions, and it now con- 

 tains within its mass remnants of what were apophyses from the top 

 of the intrusions. 



