BASAL GNEISSES OF THE HIGHLANDS 



Tabulation of the formations 



377 



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SEDIMENTARY SERIES 



IGNEOUS SERIES 





Unconformity 



Cortlandt series 



Lower Siluric 



Hudson River slates 

 Wappinger limestone 



Diorites 

 Gabbros 

 Peekskill granite 







Cambric 



Poughquag quartzite 

 Unconformity 



Harrison diorite 





Pegmatites 





Manhattan schist 

 Inwood limestone 



Basaltic and diabasic or 



dioritic intrusions 

 Granite and other dikes 



Precambric 



Overlap unconformity 







Grenville series 



Lowerre quartzite, Ford- 

 ham gneiss, and the 

 various basal gneisses of 

 the Highlands, including 

 interbedded limestones, 

 quartzitic and graphitic 

 schists 



Storm King granite 



Yonkers gneiss and other 

 granites, diorites, and cor- 

 responding gneisses 



Relative age of the igneous intrusions 



All igneous rocks of this area are younger than the sedi- 

 mentary members of the basal gneiss. But there are great 

 differences among them. Some of the intruded stringers and 

 sills may date back to the early history of these sediments 

 since they partake of all the metamorphic changes that char- 

 acterize these ancient strata including recrystallization and 

 flowage. Representatives of such early types are mostly granite 

 gneisses and are everywhere confined to the basal gneiss for- 

 mation. Many of them are very similar to the coarser meta- 

 morphosed sediments and lead to the greatest uncertainties of 

 interpretation. Others, such as the pegmatite streaks and some 

 of the basic intrusions of original diabasic character, belong to 

 the period of most extensive metamorphic activity and penetrate 

 also the next overlying limestone (Inwood) and the schist (Man- 

 hattan). But they do not affect anything higher. Therefore 



