TITLES AND ABSTRACTS OF PAPERS 93 



PEGMATITE, SILEXITE, AND APLITE DIKES OF NORTHERN NEW YORK 

 BY WILLIAM J. MILLER 



(Ahstract) 



Acidic dikes of various types are abundantly developed in many portions of 

 the Adirondack syenite-granite series. More recently it has become evident to 

 the writer that certain generally accepted interpretations are not satisfactory 

 when applied to these acidic dikes. It is proposed to use the term "silexite" 

 for all masses of pure, or nearly pure, silica genetically related to pegmatite or 

 aplite and usually of dike origin. 



Pegmatite, silexite, and aplite dikes are wonderfully displayed in almost 

 countless numbers in the granites of the Lyon Mountain quadrangle. Some of 

 the more important conclusions resulting from their study are as follows: (1) 

 by far most of the pegmatites consist simply of quartz and potash feldspar, 

 with minerals indicating the former presence of mineralizers other than water 

 vapor almost entirely absent; (2) pegmatite and silexite dikes are very abun- 

 dant in a fine to medium to fine-grained facies of the granite and relatively 

 rare in a coarse-grained facies, while the aplite dikes appear to be practically 

 confined to the coarse granite; (3) both pegmatite and silexite masses began 

 to develop while the granite magma still possessed a very considerable degree 

 of fluidity, and they continued to form until the enclosing granite almost, or 

 possibly completely, solidified; and (4) the aplite dikes were formed during 

 a late stage of consolidation of the enclosing granite, but none as early as the 

 earliest pegmatite and silexite masses and probably none as late as the latest 

 pegmatites. 



Acidic to rather basic dikes are commonly associated with many of the small 

 gabbro stocks of distinctly later age than the syenite-granite series in the 

 Adirondacks. Detailed studies within the North Creek quadrangle have led to 

 the following conclusions: (1) acidic masses ranging from pegmatites rich in 

 quartz and potash feldspar and with or without tourmaline, to i^egmatite which 

 is mostly potash feldspar, to basic pegmatite consisting chiefly of plagioclase 

 with more or less hornblende, to aplite dikes, and to even nearly pure quartz 

 (silexite), developed as satellites of the stocks of normal medium-grained 

 gabbro; and (2) development of the pegmatites began w^hile the gabbro was 

 still notably fluid and continued until it had almost or completely solidified. 



Presented in abstract by the author from notes. 



Discussed by Mr. Arthur Keith, Prof. J. P. Iddings, and Dr. Whitman 

 Cross. 



MAGNETIC IRON-ORE DEPOSITS OF CLINTON COUNTY, NEW YORK 

 BY WILLIAM J. MILLER 



{Abstract) 



First, the magnetic deposits, including those of the Lyon Mountain mines, 

 will be considered from the standpoint of mineral resources for war needs and 

 post-war reconstruction. Emphasis will be placed on the extent of the ore 



