10 NEW YORK STATE MUSEUM 



furnishes a clew to the derivation of such amphibolites, in distinction 

 from those which are of sedimentary origin. 



Syenite. The Adirondack syenite constitutes an abnormal 

 variety of that rock, and was not recognized as such until recently. 

 Mineralogically it occupies a middle place between the gabbroic 

 rocks and the granites. A green augite is nearly always the chief 

 dark constituent, but hornblende and hypersthene may be present. 

 The feldspar is commonly microperthite. Orthoclase, oligoclase, 

 quartz and magnetite are the more important of the other minerals. 

 The rock nearly always has a greenish color, varying from light to 

 dark shades. When there is a considerable proportion of the dark 

 constituents, it resembles the gabbro so much as to be hardly dis- 

 tinguished in the field, a resemblance which is even closer in com- 

 paring gneissoid varieties of the two rocks. With the increase of 

 those minerals there is apt to be a change also in the feldspars 

 shown by the preponderance of the oligoclase over the alkali feld- 

 spars. It is not apparent, however, that the syenite ever merges 

 completely into the gabbro, the evidence tending to show that the 

 two are separate and distinct intrusions. On the other hand the 

 acid types of the syenite pass into typical granite, as was first 

 demonstrated by Professor Smyth. 



The syenite occurs in local intrusions all through the Adiron- 

 dacks outside of the anorthosite area. It is developed in great 

 force in the northern section, specially in Franklin county, and is 

 common in the eastern part though the different areas here have 

 not yet been delimited. On the south side the Precambric outlier 

 near Little Falls consists of syenite. The Diana-Pitcairn area on 

 the northwest, described by Professor Smyth, deserves mention 

 as affording the first evidence of the intrusive character of the 

 rock and its lithologic relations. 



Granite. The granites, with the derived gneisses, are the most 

 frequent of all the intrusives. They are closely involved with the 

 Grenville series and over large districts are the only igneous forma- 

 tion present. It has already been pointed out that they constitute 

 an important factor in the belt of Saranac gneisses. 



The granites are prevailingly light colored, gray or reddish rocks. 

 Feldspar and quartz always predominate and may be practically 

 the only minerals present. Hornblende granite seems to be more 

 common than the mica varieties, while augite granite occurs as a 

 variation of the syenite. It is only rarely that the intrusions have 

 preserved their original massive character, a granulated cataclastic 

 and gneissoid texture being the rule. In regions where compression 



