GEOLOGY OF THE BROADALBIN QUADRANGLE 2^ 



sthene ; 1 5 per cent hornblende ; 5 per cent biotite ; and 5 per cent 

 magnetite. This rock is a non-metamorphosed, medium grained 

 hypersthene gabbro or norite which, in every way, greatly resembles 

 the dike gabbros of the North Creek sheet now being studied by 

 the writer and which, in that region, seem to be older than the 

 fine grained diabases. 



A slide from the basic dike rock one mile north-northwest of 

 Northville contains: 35 per cent plagioclase (labradorite with some 

 andesine) ; 15 per cent orthoclase (some with albite twinning) ; 20 

 per cent hypersthene (pale green to reddish brown pleochroism) ; 

 25 per cent hornblende ; 5 per cent magnetite ; and a little pyrite 

 and zircon. The rock shows a fine grained granitoid texture 

 which, because of its unusual composition, should be called a horn- 

 blende-orthoclase-hypersthene gabbro or norite. At this locality 

 there is not a single dike but rather a number of small branching 

 tongues which are beautifully shown in relation to the syenite. 

 Some of the branches cut through the syenite very irregularly while 

 others are perfectly parallel to the gneissic bands. There is no 

 sign of contact metamorphism along the very sharp contacts with 

 the greenish gray syenite. 



PALEOZOIC ROCKS 



The Paleozoic formations, which are all of Cambric and Or- 

 dovicic ages, occupy about three-fifths of the area of the quadrangle. 

 Because of their distinct stratification, fossil content, and lack of 

 metamorphism they present sharp contrasts to the Precambric 

 rocks. These strata have been little disturbed by folding or tilting 

 except near the faults where the dip is often pronounced. Because 

 of the faulting the general Paleozoic dip can not be well determined, 

 but from Barkersville southward it is something like seventy-five 

 or eighty feet per mile southwestward. 



POTSDAM SANDSTONE 



The Potsdam sandstone, which is of upper Cambric age, is the 

 oldest Paleozoic formation of the district. It everywhere rests 

 upon the Precambric, being separated from the ancient gneisses 

 by a profound unconformity. The absence of Lower Cambric 

 strata here and their presence along the eastern border of the 

 State clearly shows that the Cambric sea encroached upon the land 

 from the east. Speaking of the Potsdam sandstone of the Adiron- 



