24 NEW YORK STATE MUSEUM 



GABBRO AND DIABASE DIKES 



With one or two possible exceptions, these black dike rocks rep- 

 resent the latest igneous activity of the district. Some are true 

 gabbros while others are diabases. In all, nine of these dikes — 

 four west of Northville, four in the vicinity of Batchellerville and 

 one at Barkersville — have been noted, but more than likely others 

 occur in the woods. The dike at Barkersville is somewhat 

 gneissoid and for this reason is rather doubtfully classed with the 

 other eight, all of which appear to be entirely devoid of metamor- 

 phism. In all cases the rock is hard and fresh, due to the fact that 

 the decomposed material has all been removed by ice erosion. 

 These basic rocks are certainly younger than the Grenville, syenite, 

 or granite since these latter have all been cut by the dikes. Also 

 the eight non-metamorphosed dikes were certainly intruded after 

 the cessation of the pressure which produced the foliation of the 

 Precambric rocks. That they are of Precambric age has not been 

 proved within the quadrangle itself, but their close similarity to such 

 rocks occurring in the Adirondacks, and the fact that no such 

 rocks have been seen cutting the Paleozoic in this part of the State, 

 leaves little room for doubt regarding their Precambic age. The 

 gneissic structure of the Barkersville dike suggests that it is older 

 than either the nonmetamorphic basic dikes or the pegmatite, but 

 its mineral composition is almost precisely like that of the Batchel- 

 lerville dikes. 



The variations in these rocks will perhaps be best shown by 

 describing several types. A thin section from the dike one mile 

 northeast of Batchellerville shows : 60 per cent lath-shaped plagio- 

 clase (andesine to labradorite) ; 25 per cent hypersthene (faintly 

 pleochroic) ; 10 per cent biotite (much changed to chlorite) ; and 5 

 per cent magnetite (much changed to leucoxene). This rock is a 

 true hypersthene diabase with the ophitic texture beautifully shown. 

 The dike is about 400 yards long and of varying width up to 100 

 feet. The rock is fine to medium grained, weathers brown on the 

 immediate surface, shows no sharp contacts, and strikes parallel 

 to the general foliation of the Grenville. In close proximity to 

 the dike the Grenville is usually badly twisted, probably due to the 

 force of intrusion of the molten rock. The composition of the 

 dike at Barkersville is almost exactly like this except for a little 

 hornblende and pyrite. 



The dike three-quarters of a mile west-northwest of Northville 

 shows about: 50 per cent basic plagioclase; 25 per cent hyper- 



