102 NEW YORK STATE MUSEUM 



General Description 



The southeastern half of the Lake Bonaparte and most of the 

 Lowville quadrangles are underlain by a banded or belted gneissic 

 complex. The composition of these belts ranges from a siliceous 

 granite through grano-syenite, hornblende syenite and augite syenite, 

 to a basic augite-hypersthene syenite. The belts are long and 

 narrow, usually varying from one-half of a mile to several miles in 

 width. Blocks of Grenville gneiss occur as scattered inclusions or as 

 a host of separated fragments here and there. The belts possess in 

 general a northeast-southwest trend but with conspicuous deviations 

 in the northwest corner of the Lowville area and the southwest 

 corner of the Lake Bonaparte quadrangle, where the bands strike 

 east-west and northwest-southeast. The banded or belted nature 

 of the syenite-granite gneissic complex is indicated in a generalized 

 way on the accompanying map. Toward the northwest the igneous 

 gneisses are bordered by the Grenville series, comprising gneisses 

 and limestones intruded by granites which extend to the St Lawrence. 

 On the southwest they are overlain unconformably by sedimentary 

 beds of Paleozoic age. 



The different rocks of this syenitic complex may be roughly 

 grouped as belonging to two masses, the members of each of which 

 possess some characters in common which are different from the 

 other. Thus the rocks of what has been called the Diana mass 

 uniformly have a coarse porphyritic texture with phenocrysts up to 

 i or 2 inches in diameter, and when examined in thin section show a 

 partial granulation or pulverization (plates 2 and 3). The mem- 

 bers of the other body, named the Croghan mass, are uniformly 

 medium equigranular and in thin section show no signs of crushing 

 whatever (plate 1). Each mass, however, has an equally wide 

 variation in composition. But the Diana mass is on the whole one 

 step more basic than the Croghan. Thus, the members of the 

 former body range from an augite-hypersthene syenite to a horn- 

 blende grano-syenite, whereas the latter ranges from an augite 

 syenite to an acid granite. The area occupied by each mass is 

 indicated on the map by the boundary line separating the crushed 

 from the uncrushed gneisses ; the Diana body lying to the northwest 

 of the line and the Croghan mass to the southeast. All the members 

 of both masses, however, possess a foliation equally well developed. 

 Both bodies are cut by veins of granitic pegmatite and by hyperite 

 dikes. The latter are most abundant in the vicinity of a stock of 

 gabbro indicated on the map. 



