ELIZABETHTOWN AND PORT HENRY QUADRANGLES 



123 



Fig. 28 Old Bed ore. The blackis mag- 

 netite; the stippled mineral is apatite; the 

 lined mineral is emerald-green pyroxene. 

 Actual field o.i inch. 



The great ore body lies beneath a cap of this very acidic rock. The 

 '* 21 gneiss " appears at times at other horizons in the series but it is 

 not always accompanied by ore. 

 It is moreover very similar to the 

 wall rocks at Hammondville, if 

 not actually identical with them. 



Beneath the ore appears a more 

 basic variety, rich in hornblende, 

 augitC; and sometimes in biotite. 

 Plagioclase is abundant and inas- 

 much as massive gabbro is seen 

 beneath the Barton Hill ore, the 

 basic gneiss was believed to be a 

 metamorphosed representative of 

 it and was called " gabbro-gneiss." 

 Meantime, however, we have 

 learned much regarding the syen- 

 itic series of the Adirondacks and have also obtained some thou- 

 sands of feet of drill cores not previously seen. From the latter 

 it is evident that representatives O'f the former are the chief mem- 

 bers in the series. Many more slides of the supposed " gabbro- 

 gneiss " serve to ally it with basic developments of the syenite 

 series and it is much more defensible to consider the ores as lying 

 between the two extremes, an acidic and a basic, of the great 

 syenite series. In the basic we find so much microperthitic 

 orthoclase that it is practically impossible to draw sharp lines of 

 distinction among these varieties when starting from the normal 

 syenitic type. 



In the early paper a band of acidic gneiss was identified in the 

 hanging wall of the Barton Hill group, and was called the " Orchard 

 gneiss '*' after one of the pits. The rock was composed essentially 

 of quartz and plagioclase with now and then a few magnetites and 

 zircons. One related occurrence had microcline. This must be 

 regarded as essentially a phase of the " 21 gneiss," since in the 

 one case the albite molecule crystallized as spindles in the ortho- 

 clase, yielding microperthite, while in the other it combined with 

 a little of the anorthite molecule to yield oligoclase. The Orchard 

 gneiss is soon succeeded, as one ascends Barton hill, by a darker 

 variety containing microperthite as the most ahundant mineral, with 

 quartz, oligoclase and orthoclase as the other light colored com- 

 ponents. The dark minerals are brown hornblende, emerald-green 

 augite and rarely hypersthene. Apatite and magnetite are also 



