24 NEW YORK STATE MUSEUM 



tends greatly to disguise the relationship and to render a demon- 

 stration difticult. Igneous phenomena and their expiring effects 

 must have been very general and have probably occasioned wide- 

 spread recrystallization. Undoubtedly tney have set in migration 

 many heated solutions. 



The limestone becomes at times extremely small in amount and 

 may be represented by little more than calcareous streaks amid 

 more siliceous rocks, such as mica schist and quartzite bands. The 

 whole may be folded in a most remarkable way. The more re- 

 sistant silicates having been involved in a plastic medium like calcite 

 have been bent into shapes that seem almost beyond the power of 

 brittle minerals to assume. The presence^ however, ot the hnie- 

 stone is indicated by the pitted and cavernous weathering. 



Mica schist or schistose gneisses are known in several localities 

 which also represent the sedimentary series. The rocks are thinly 

 laminated and are much more abundantly provided with biotite than 

 are the eruptive gneisses. The banding runs regularly for greater 

 distances and reproduces the persistent bedding of sediments rather 

 than the sheared and dragged individual minerals of the eruptives. 



Besides the more schistose gneisses there are others less thinly or 

 regularly banded and yet not corresponding exactly to any of the 

 well defined eruptive rocks. Where they display sharp contrasts 

 of light and dark bands which are persis:ent over distances of sev- 

 eral feet or more and which are difficult to explain except on the 

 basis of the contrasts in composition which might arise in sedimen- 

 tation, the writer's disposition has been to group them with the 

 sedimentary types. It is realized that eruptive rocks themselves 

 do display marked banding and gneissoid structures which are due 

 to magmatic differentiation,^ but yet in open questions like those 

 presented in the Adirondacks, as a matter of opinion the writer 

 leans rather to the sedimentary interpretation especially when the 

 rocks are associated with undoubted sediments. 



Some complication arises because of the abundance of pegmatitic 

 matter even on a small scale. Injected gneisses are not unknown 

 and inasmuch as such quartzites as can be recognized in the 

 region are so thoroughly recrystallized as to simulate vein quartz 

 or pegmatitic quartz, lenticular masses of this character may at 

 times give rise to the suspicion of old sandstones. Well defined 

 quartzites are, however, much less in evidence in the two quad- 

 rangles under discussion than in several to the south. 



1 As very clearly shown by Sir Archibald Geikie and J. J, H. Teall for 

 *.he gabbros of Skye. Geol. Soc. Lond. Quar. Jour. 1894. 50:645- 



