GEOLOGY OF SARATOGA SPRINGS AND VICINITY 21 



PRECAMBRIC IGNEOUS ROCKS 



General statement. In order of age the Precambric igneous 

 rocks of the quadrangles are the l.aurentian (?) granite, the syen- 

 ite, and the trap (diabase) dikes. There are in addition very 

 small and occasional masses of amphibolite which are probably 

 original gabbros and younger than the syenite, but they merit only 

 passing notice because of their small bulk and problematic 

 character. 



Laurentian (?) granite. A previous statement makes reference 

 to a white, granitic rock, intricately involved with the Grenville 

 schists, which is a common constituent of the Grenville throughout 

 the southern Adirondacks and which we have heretofore regarded 

 as a Grenville rock. Thus Gushing, reporting upon similar rocks 

 from the Little Falls quadrangle, classes them as Grenville.^ Kemp 

 and Hill describe the similar rock from the '* Noses " in the 

 Mohawk valley as a Grenville sediment.- The problem is an 

 involved one since the granite is seldom pure but has everywhere 

 taken in a considerable amount of the adjacent Grenville rocks, 

 giving rise to a mixed rock composed of varying amounts of 

 granite and schist. The recognition of pegmatitic phases of the 

 rock threw the first doubt upon its sedimentary character; after- 

 wards it came to be recognized that the granite itself, as well as its 

 pegmatites, was intrusive as regards the schists. The chemical 

 analysis eventually settled the question. 



The granite is a difficult rock to describe because it is nearly 

 everywhere so involved with the schists, or with material from 

 the schists, as to make rock free from such contamination difficult 

 of recognition. The mica is the most obvious of the contributions 

 from the schist, which is certainly much richer in that mineral 

 than the original granite. Hence arises a tendency to infer that 

 those portions of the granite freest from mica are those least con- 

 taminated. Such portions show a very white rock composed of 

 white feldspar and quartz and a very little black mica (biotite). 

 In addition are small amounts of apatite, titanite, zircon, and 

 magnetite. Pink garnets are always present but are not regarded 

 as original minerals of the rock. They are most abundant in the 

 schists which have been thoroughly impregnated with granite, and 

 have in most cases resulted from corrosive interaction between the 

 minerals of the two rocks. In other words, most of the garnet is 



1 N. Y. State Mils. lUil. 77, p. 17-ia 



- N. V. State Gcol. i()tli Ann. Rcp't p. r3J-r35. 



