r92 NEW YORK STATE MUSEUM 



rocks. The general identity of all these minerals with those in 

 the other rocks is emphatic evidence of their relationship. 



The feldspar however differs. More than half of it consists of 

 well twinned plagioclase whose extinctionis reach 2il°, indicating 

 an acid labradorite. But there is also a large amount of the 

 curious micrographic intergrowth like that in the preceding 

 rocks. For the most part both of the feldspars- of these inter- 

 growths are untwinned, but an occasional labradorite fragment 

 shows a similar intergrowth. This renders it possible that 

 untwinned labradorite is one of the feldspars of the usual inter- 

 growth, though this is not thought likely. 



This rock is strictly a gabbro (norite), though with a syenitic 

 facies. The identity of its component minerals with those in the 

 other rock sufficiently establishes its relationship to them, how- 

 ever. No sharp line can be drawn between any of the varieties, 

 since all gradations are found, yet the rock ranges from a gran- 

 ite to a gabbro in composition. In the frequency and amount of 

 such change it is quite like the syenite of the Adirondack region, 

 but, considering the slight areal extent of the exposures here, 

 the amount of change is surprising. The accessibility of the 

 locality and the excellence of the exposures combine to render it 



well worth a visit. 



Chemical composition 



A thorough chemical study of the Little Falls rocks could not 

 fail to furnish results of great interest, though the writer is 

 strongly of the opinion that the table of analyses would be quite 

 similar to that of the various syenites given in the preceding 

 report. 



The single analysis so far available is that of the ordinary augen 

 syenite at the village (analysis 13 of the preceding report). A 

 precise calculation of this rock can not be made because of the 

 unexplained failure of the ferrous iron determination. Assuming 

 that Ifo of magnetite is present, which can not be greatly wide of 

 the mark, that the alkalis are wholly in the feldspar, and that 

 the excess of alumina is in the hornblende and excess of lime in 

 the anorthite molecule, the calculated composition is: 



