36 NEW YORK STATE MUSEUM 



The origin of the gneisses is very obscure. In some respects they 

 appear to be largely igneous in character. In many places their 

 sedimentary origin seems almost certain. It is entirely possible 

 that the two kinds occur together in a parallel and roughly alternate 

 arrangement, but faulting makes it impossible to decide this point 

 in the face of the other difificulties present. The thickness is too 

 great to permit the interpretation of a monoclinal series. 



It seems entirely justifiable to attribute the apparent igneous 

 character to profound metamorphism. It is plain that if the 

 gneisses represent a sedimentary series in any part, the strata must 

 have been jammed into close folds and overturned. If folding 

 was accompanied by the injection of igneous rocks along the axes 

 of the anticlines, the accompanying alteration would have been very 

 severe and both sedimentary and igneous types would have come 

 strongly to resemble each other. There would probably be no dis- 

 tinguishable exomorphic and endomorphic effects to aid in separat- 

 ing the two. 



The gneisses below the bridge in the Mount Beacon brook section 

 show a " bedded " character more clearly than at any other place. 



The general absence of crumpling and crinkling in the gneisses is 

 noteworthy in considering the possibility of their sedimentary origin. 



Interbedded limestones, if such could be found, were thought 

 of as likely to afford the most convincing evidence of a sedimentary 

 series in these gneisses. Dr C. P. Berkey has discovered such lime- 

 stones in the Highlands farther south^ and in the Fordham gneiss of 

 New York city.- The possibility that the basic rock and bastite 

 ledges at Hortontown, described in the following pages, might be 

 altered calcareous and magnesian sediments of Precambric age 

 was considered, but the field relations do not easily permit this 

 interpretation. 



Taken as a whole, the gneisses in this quadrangle present sufficient 

 diversity to be considered, at least in part, as an altered sedimen- 

 tary series. 



NAME AND CORRELATION 



Dr C. p. Berkey^ has correlated the basal member of the Man- 

 hattan series with the basal gneisses of the Highlands and has 



1 Structural and Stratigraphic Features of the Basal Gneisses of the 

 Highlands. N. Y. State Mus. Bui. 107, 1907. 



2 Science, n. s., 37 '.936. 



^ Structural and Stratigraphic Features of the Basal Gneisses of the 

 Highlands. N. Y. State Mus. Bui. 107, 1907, p. 361. 



