WRioHT-FEXNER] PETEOGRAPHIC STUDY 73 



labradorite), quartz, liornblende, pyroxene, magnetite, colorless vol- 

 canic glass of low refractive index, and argillaceous material. On 

 immersion in water this earth crumbles sliglitly. Its water solution 

 contains abundant cliloride (tested with silver nitrate). 



Part of the earthy material which surrounds the scoriae in this 

 specimen was heated to 1,100° for 30 minutes; then it remained at 

 1,200° for 3^ hours, and finally at 1,100° for U hours. The product 

 was a vesicular, purple-colored glass, in which the original fragments 

 of quartz and plagioclase were still visible under the microscope. In 

 transmitted light the glass appeared colorless, but was filled with fine, 

 dust-like particles of hematite. No new development of crystals was 

 observed. The refractive index of the glass was about 1.51. The 

 scoriaceous part of the specimen was heated at 1,240° for 1 hour and 

 then held at 1,000° for 1^ hours. The resulting glass was greenish- 

 brown in color and had evidently settled quietly in the crucible. 

 Under the microscope numerous grains of the original quartz and 

 plagioclase fragments were observed. Clusters of acicular microlites 

 of medium birefringence, parallel extinction, and refractive index 

 greater than 1.53 are abundant, but are too fine for satisfactory 

 determination. The refractive index .of the glass, which appears 

 pale-brown in transmitted light, varies from about 1.515 to 1.54. 



No. 263727. Specimen label. '^Locality: MiraTnar. Material: 

 Tierra cocida and scorise comhined. Contact transition." 



The hand specimen shows a regular and uniform transition from a 

 dark-gray scoria filled with small vesicles to a brick-red material, 

 which bears a close resemblance to some of the specimens of baked 

 earth. It is different from the latter, however, in tliis respect, that, 

 while the baked earths have a close, compact texture, the portion 

 of this specimen which resembles them most (observed with a 

 binocular magnifying glass, magnification 65 diameters) is filled 

 with minute holes and is distinctly glassy in character. A thin 

 section wliich was cut across both the black scoria and the transition 

 zone to the bright-red material bore out tliis observation. The 

 black scoria is filled with fragments of quartz, plagioclase (acid 

 oligoclase to basic labradorite), pyroxene, and magnetite, set in a 

 glassy matrix in wliich a few microlitic crystals have developed, but 

 which is essentially an undifferentiated glass except for a black dust, 

 probably of magnetite. On passing toward the compact red portion 

 the principal change observable is the replacement of the black dust 

 by red dust, which is more prominent and tends to cover up the 

 crystallized fragments and even the minute vesicles. In the more 

 compact red portion the red iron oxide is so prominent that the 

 mineral fragments are almost veiled to view. A careful determina- 

 tion of the mineral fragments in the black and the red portions of the 

 specimen proved them to be of the same general size and kind, chiefly 



