GEOLOGY OF THE BROADALBIN QUADRANGLE 23 



PEGMATITE DIKES 



Numerous pegmatite dikes have been found cutting through the 

 Grenville, syenite, and granite. A few of the larger and more 

 accessible ones are located as follows : At the feldspar mine two 

 and one-half miles north-northeast of Batchellerville ; two miles 

 north of Edinburg; two miles northeast of Northville; and about 

 two miles west of Sacandaga Park. These dikes are apparently 

 all nonmetamorphosed and they cut through the country rock in 

 very irregular shaped masses and stringers. A description of the 

 pegmatite at the feldspar mine north of Batchellerville will be fairly 

 illustrative of the other occurrences. This rock has recently been 

 described by Mr E. S. Bastin of the U. S. Geological Survey and 

 the following extracts are from his report :^ " The rock is a granite 

 pegmatite which has been worked from two open pits. Quartz 

 occurs in pure masses several feet across and also in graphic inter- 

 growth with feldspar. The feldspar is light gray microcline, 

 finely intergrown with small amounts of albite. It occurs in pure 

 masses, the largest four feet across, and the feldspars of some of 

 the coarser phases of the graphic granite are three feet across. The 

 finer grained parts of the pegmatite contain ' books ' of muscovite 

 oriented in every direction.- Biotite is not abundant, but one flat 

 crystal observed was four feet long and three feet wide. Beryl, 

 of dark blue-green color, translucent to transparent, is moderately 

 abundant. The pegmatite for two or three feet next to the contact 

 of the pegmatite with the schist at this pit is an irregular or ar- 

 borescent intergrowth of quartz and feldspar inclosing some large 

 muscovite ' books.' The pegmatite is intrusive, with sharp con- 

 tacts with a light gray to dark gray quartz-biotite-feldspar gneiss 

 of variable character. The contact in some places parallels the 

 foliation and in other places cuts sharply across it." 



Whether the pegmatite is older or younger than the black basic 

 dikes below described has not been determined within the quad- 

 rangle. However, just beyond the map limits, and in an abandoned 

 feldspar mine two and one-half miles west-northwest of Cranberry 

 Creek, a pegmatite and a basic (diabase?) dike may be seen in 

 sharp vertical contact and the basic rock cuts the pegmatite thus 

 proving the greater age of the latter in this case at least. 



1 Feldspar Deposits of the United States, U. S. G. S. Bui. 420. 



