GEOLOGY OF THE VICINITY OF LITTLE FALLS 17 



Since these are small outliers and consist practically wholly 

 of s.yenite, they can furnish no decisive evidence of the age of the 

 syenite as compared with that of the other pre-Cambrian rocks. 

 It has been stated that the Grenville rocks are closely involved 

 with apparent igneous rocks which seem to have been either con- 

 temporaneous with them or to have been intruded into them not 

 long after their deposition. Also that at a later date there was a 

 time of great igneous activity in the region, when huge masses 

 of molten rock were intruded into the Grenville rocks ; and that 

 at a third and much later time there was a further renewal of 

 igneous activity, though in a minor degree. The writers dis- 

 position is to regard the syenite under discussion as dating from 

 the second of these periods, and as of much the same age as the 

 the great syenite, anorthosite and gabbro masses of the central 

 and eastern Adirondack region, but it should be emphasized that 

 there is no decisive evidence in proof of this view. These two 

 small areas are likely connected underneath and represent por- 

 tions of the surface of the same mass, and it certainly represents 

 a different intrusion from those in the heart of the woods, though 

 regarded as belonging to the same group of intrusions. 



Diabase dike. The only representative of the third igneous 

 period which has been discovered within the map limits, is a huge 

 dike of the rock known as diabase, which is exposed about a half 

 mile east of the Little Falls depot along the Dolgeville Kailroad. 

 The rock is black and fine grained, with many half inch, por- 

 phyritic feldspars, of a general greenish gray, dull appearance 

 because of alteration. Xear the edges of the dike the rock be- 

 comes very black and dense and of stony texture, because of more 

 rapid cooling and solidification there, due to the chilling effect 

 of the walls. The dike is at least 120 feet in width, an unusually 

 large size for these Adirondack dikes. 



Grenville rocks. In the main pre-Oambrian exposures within 

 the ma]i limits, rocks which are of a])})arent sedimentary origin, 

 and hence classed as of Grenville age, play an important part. 

 The extreme metamorphism which they have suffered has pro- 

 duced complete recrystallization, with consequent disappearance 



