226 Pogue, Jr. — Geology and Structure of Volcanic Bocks. 



smooth and of a light gray or white color. In places contorted 

 and wavy lines, indicative of fiowage, are visible. The rock 

 is brittle and breaks with a conchoidal fracture into pieces 

 with sharp, translucent edges. Upon fresh fracture, it appears 

 black, dark green, or grayish green; with feldspar phenocrysts 

 uniformly, though not abundantly, distributed. Some phases 

 are exceedingly dense and can be distinguished with difficulty 

 from the highly silicified fine tuff. 



The analysis of the rock, made by the writer, is as follows : 



Si0 2 Al a 3 Fe a 3 FeO MgO CaO Na 2 K 2 H 2 C0 2 =Total 

 74-55 10-75 1-24 2-11 trace 1'50 5-39 2-70 0-61 1*30 100-15 



From this, its position in the quantitative system is cal- 

 culated to be : Class I, persalane ; order 4, quardofelic ; rang 

 2, domalkalic ; subrang 4, dosoclic. The rock, therefore, cor- 

 responds to lansenose. 



The rhyolite has been completely devitrified since its con- 

 solidation. According to the nomenclature proposed by 

 Bascom,"* it is an aporhyolite. 



Dacite. — Dacite composes the hill east from Cid, known as 

 Kemp Mountain. It forms here an area of oval outline, about 

 1 mile long and j- mile wide. The rock is distinctly massive 

 and occurs in numerous, grayish-white, rounded masses, 

 resembling the outcrops of rhyolite. In hand specimen, it is 

 rather tough, grayish green in color, and has a slightly mottled 

 surface, due to a few feldspar phenocrysts and to specks 

 and small patches of biotite and chlorite. A close inspection 

 reveals that many of the feldspar crystals are green in color 

 from a slight admixture of epidote. 



A feature of interest, brought out by the microscope, is the 

 fact that quartz does not occur as phenocrysts. It is found, 

 however, in the groundmass as abundant small crystals of fair 

 rhombic outline. Similar dihexahedral quartz crystals, show- 

 ing a rhombic cross section, have been described by Kiichf as 

 occurring in the groundmass of a dacite from South America. 



The analysis of the rock, made by Dr. A. S. Wheeler of the 

 University of North Carolina, is as follows : 



Si0 2 A1 2 3 Fe 2 3 FeO MgO CaO Na 2 K 2 H 2 C0 2 Total 

 72-33 14-56 0-15 2-22 0-91 2-55 3-40 2-82 0-30 0-00=99-24 



In the quantitative system the rock is lassenose : Class I ? 

 persalane; order 4, quardofelic ; rang 2, domalkalic ; subrang 

 4, dosoclic. 



* Bascom, F., The Structure, Origin and Nomenclature of the Acid 

 Volcanic Rocks of South Mountain, Pennsylvania. Jour. Geol., vi, 

 p. 827, 1893. 



fKiich, Richard, Petrographie [from Oeologische Studien in der 

 Republik Colombia], 1892, p. 69. 



