318 WATSON AND CLIXE IGNEOUS DIKES IN VIRGINIA 



Out of a total of -16 rocks listed in Washington's 13 tables as miaskose. 

 10 are nepheline syenites. 



TESCHEXITE OR AXALCITE-BASALT (MOXCHIQUOSE ( ?) ) 



Megascopic character. — The rocks included under this name are very 

 dark gray, almost black in color, and usually porphyritic in texture, 

 although some specimens show very few phenocrysts. Augite and biotite, 

 usually developed with crystal boundaries, and the former always in 

 excess of the latter, are the mafic minerals occurring as phenocrysts. 

 Though less abundant, biotite is more conspicuous as a phenocryst and 

 occurs in larger crystals than augite. The largest biotite crystals attain 

 a size of 1 centimeter in diameter, but most of them average about 1 

 millimeter. Crystals of augite averaging about 3 millimeters in length 

 exhibit perfectly developed prismatic and pinacoidal faces. The ground- 

 mass in which these phenocrysts are set is fine granular, but on close 

 examination both dark and light colored minerals are distinguishable, 

 the former always in excess. With the aid of a pocket lens, a fair 

 abundance of the light minerals can be detected occupying positions be- 

 tween the dark constituents. 



One of the most conspicuous megascopic features of some hand speci- 

 mens of the rock is the large amount of calcite and flesh-colored analcite 

 occupying spherical-shaped areas somewhat abundantly scattered through 

 the rock. Sometimes calcite alone completely occupies the area, less 

 often analcite : but as a rule the two minerals occur together in the same 

 area, when calcite forms the center and analcite the rim or border. The 

 areas rarely exceed 1.5 centimeters in diameter, with an average of about 

 5 millimeters. These areas probably represent phenocrysts and are 

 similar to those described in detail under camptonite. 



Microscopic character. — In thin-sections under the microscope the fol- 

 lowing primary minerals are seen : Augite, biotite, magnetite, plagioclase 

 and orthoclase feldspar, analcite, and a little apatite. All thin-sections 

 examined show alteration, the secondary minerals named in order of 

 abundance being calcite. zeolites, chlorite, muscovite, and pyrite. 



One of the most abundant of the original colorless minerals, analcite. 

 has been completely altered in most of the thin-sections into cloudy 

 aggregates of minute particles of intimately associated calcite, muscovite. 

 and zeolitic substance* Some of the cloudy areas have hexagonal out- 

 lines, some rectangular; but most of them are irregular in outline. The 

 mineral originally occupying these positions in the rock was the least 

 resistant one to weathering- and it was doubtless analcite. Two va- 



« Professional Paper No. 14, U. S. Geological Survey, 1903, pp. 207-213. 



