GEOLOGY OF OGDENSBURG 21 



so, feldspar crystals of an inch or more in length being not uncom- 

 mon. They are always prominentlygneissoid, with a conspicuous 

 development of black mica, the feldspar crystals are strung out 

 parallel to the foliation, and are themselves often granulated or 

 mashed, sometimes completely, sometimes only partially. The 

 mashed feldspars are always red, while the unmashed are lighter 

 colored, usually gray. The rock is therefore usually a mottled 

 looking, red and black rock, the drawnout, mashed feldspars fur- 

 nishing the red constituent, and the intervening, fine-grained, 

 micaceous portions the black. The rock is not so siliceous as the 

 usual granite of the region, usually running about 68 per cent of 

 silica as against some 72 per cent for the other. There is con- 

 siderable quartz in the rock, but probably the quartz and much of 

 the mica develop from recrystallization of original feldspar during 

 metamorphism. 



As they occur on the Ogdensburg sheet, the distribution of these 

 syenite masses is peculiar. They occur as a series of disconnected, 

 tonguelike masses, drawn out parallel to -the general trend of the 

 neighboring Grenville rocks through which they cut. Eight such 

 separate tongues are mapped within a comparatively small area on 

 the southern part of the Ogdensburg sheet, and there are others to 

 the southwest on the Gouverneur and Hammond sheets. They 

 have not been traced to any connection with a mass of ordinary 

 syenite. Such a mass may be found on these sheets when they are 

 mapped, from which these tongues may be dikelike offshoots. But 

 they do not produce the impression of dikes, nor are their contacts 

 with the Grenville of the type which would normally come with 

 dikes. These contacts are not sharp, but exceedingly blurred. The 

 adjacent Grenville rocks are cut by a multitude of narrow dikes 

 running out from these tongues, so that there is a considerable 

 intermediate zone of mixed rock around each one of them. The 

 relations suggest the influence of a large mass of hot molten rock, 

 much larger than indicated by the present surface exposures. It 

 is therefore quite possible that these tongues are mere upshoots 

 from a large mass of syenite beneath, hidden from view by the 

 Grenville cover, but not far beneath the present surface. This sug- 

 gestion is, of course, speculative, but would explain the present 

 surface relations, and is opposed by no facts known to us. 



Diabase. Only one trap dike has been seen within the mapped 

 limits. This is found in Dekalb, about 2 miles south of Kindrews 

 Corners and just west of the Oswegatchie. It is a large dike : 45 



