314 WATSON AND CLINE IGNEOUS DIKES IN VIRGINIA 



NEPHELINE SYENITE (MIASKOSE) 



Megascopic character.— Map, figure 1, shows two lines of outcrop of 

 nepheline syenite, which for convenience of description may be desig- 

 nated the north dike and the south dike, located 3y 2 miles from each 

 other. For purposes of description this designation into a north and 

 south dike is important, since the rock in the two localities exhibits 

 marked differences in texture and mineral composition. 



The rock of the north dike, which is prominently developed on the 

 Glick farm, is dove-colored when fresh, strongly resembling many of the 

 associated limestones of the Shenandoah group, for which it might easily 

 be mistaken unless carefully examined. It is porphyritic in texture, with 

 a medium-grained ground-mass, in which a sprinkling of orthoclase crys- 

 tals, a few hexagonal plates of biotite, and an occasional crystal of horn- 

 blende are developed as phenocrysts. The orthoclase is colorless to light 

 gray, has glistening cleavage surfaces, and is idiomorphic in outline. Its 

 crystals attain a maximum of 5 millimeters in length and 4 millimeters 

 in width. The few glistening brown plates of biotite average from 2 to 3 

 millimeters in diameter, while the occasional phenocrysts of greenish- 

 black hornblende have a maximum length of 7 millimeters. Partially 

 weathered specimens of the rock frequently exhibit a mottled appearance 

 from light gray, irregular areas of dull earthy luster and bluish cast. 



The rock of the south dike (plate 7, figure 1) is medium-grained and 

 of light gray color, with a pronounced granitic habit. Its most abundant 

 megascopic constituents are the light-colored minerals, feldspar and 

 nepheline, which can usually be distinguished from each other in hand 

 specimens. The feldspar is generally developed in small prismatic crys- 

 tals and formless grains, with good cleavage, while the nepheline is devel- 

 oped in irregular grains without noticeable cleavage and possesses an oily 

 luster. Fragments of the latter when isolated from the rock and tested 

 fused easily before the blowpipe and were readily soluble in hydrochloride 

 acid, which on evaporation yielded copious gelatinous silica. Similar 

 results were obtained from tests made on fragments of the rock from the 

 north dike. The only other visible megascopic constituents are a green- 

 ish-black hornblende developed in fair abundance in small anhedra, the 

 largest of which measure 3 millimeters in length, and occasional crystals 

 of black biotite. 



Weathered surfaces of the nepheline syenite from the south dike are 

 quite interesting in affording beautiful illustrations of differential weath- 

 ering of the essential rock minerals. The hornblende and feldspar stand 

 out in prominent relief, while the removal by weathering of the less re- 

 sistant nepheline produces a characteristic pitted surface. The protru- 





