72 BUREAU OF AMERICAN ETHNOLOGY [bill. 52 



Even the small scoria pellets in the earth frequently impress the 

 observer as presenting in large measure the original glass surface. 

 These relations indicate that the scoriae were not derived from a 

 large, compact lava mass. Microscopically the scoria is essentially a 

 microlite-bearing glass crowded with fragments of plagioclase, quartz, 

 pyroxene, and magnetite. The refractive index of the glass varies 

 rapidly even m short distances and ranges from above 1.55 to below 

 1.52. Near the microlites which have crystallized from the glass, 

 the color of the glass is often a pale-brown. The microlites appear 

 in radial spherulites and irregularly grouped acicular crs'stals. They 

 are colorless to pale-brown, of medium birefringence and of average 

 refractive index, slightly greater than 1.65. The lath-shaped indi- 

 viduals often show high extinction angles; c : jc=40'° was measured 

 in one instance. The optical character is positive. The optical 

 properties agree, so far as determined, with those of pyroxene. This 

 microlitic pyroxene does not agree in its properties with the larger 

 green pyroxene crystal . fragments noted above. The large, f rag- 

 mental crystals never show normal crystal outlines but are always 

 broken and irregular in outline. They are similar in shape, size, 

 and composition to the crystals found in the earthy loess of the 

 country. They often appear partially resorbed by the magma. 

 Some of the crj^stals are even fractured. At certain pomts the thin 

 section is crowded with angular fragments, each one of which repre- 

 sents a portion of a single crystal, and these are set in a paste of glass 

 in which there has been very little crystallization. The occurrence 

 of these fragments of plagioclase and also of cjuartz in a glass out of 

 which they were not observed to recrystallize is good evidence of the 

 melting-down of originally crystallized material. The variation in 

 refractive index of the glass and also in adjacent plagioclase frag- 

 ments (observed values ranged from 1.55 to 1.565) is evidence that 

 the rock is not a volcanic rock of any normal tj'pe. In a cooling 

 magma or lava the composition of the residual glass is fairly uniform, 

 whereas in the present rock the refractive index (and consequently 

 the composition)' of the glass varies rapidly. Different explanations 

 may be offered for the formation of such a rock type, all of which 

 involve a partial melting-do\vn or assimilation of originally crj^stal- 

 lized material containing fragments of quartz, plagioclase, pyroxene, 

 and magnetite. It is noteworthy in this coimection that no frag- 

 ments of original rock- types were observed, only isolated mineral 

 fragments such as occur in the fine earths of this region. These 

 fragments appear embedded in a glass base out of which radial 

 pyroxene crj'stals are formed. The evidence at hand is not sufficient 

 to determine definitely the mode of formation of these scoriae. 



The earth surrounding the scoria is of the usual fine, evenly grained 

 type and consists of fragments of plagioclase (oHgoclase, andesine. 



