WRIGHT-FENNER] PETROGRAPHIC STUDY 95 



condition of the loess is another proof that these particular scoriae 

 were not formed in situ. The scoriae of specimen 263731 do contain, 

 however, in some of the vesicles red baked loess, which, in turn, is of 

 a different composition from the friable brown loess adhering to 

 other parts of the same specimen. This indicates again transporta- 

 tion of the scoriae; also that at some period in their formation these 

 scoriae came in contact with the loess and that the temperature at 

 that time was between 850° and 1,050°. No evidence was found in 

 this specimen that the scoria had melted down the loess. 



Taken as a whole the evidence recorded above proves that the 

 scoriae are not normal volcanic scoriae. They are not lava in the 

 ordinary sense of the word. But they have been melted down under 

 conditions which protected them from oxidation; they were not 

 melted down, in short, in the open air, otherudse the iron oxide would 

 have passed largely into red hematite as in all the experiments cited 

 above. The temperatures and quantity of heat required for melting 

 the large masses of Necochea scoriae postulate long-continued heating 

 at a very high temperature, much higher, in fact, than is possible in 

 the open air under ordinary conditions. This fact, together with the 

 observed lack of oxidation of the scoriae, precludes the possibility that 

 they have been formed by the melting down of loess by bonfires or 

 any type of fire in the open air. 



The microscopic and thermal evidence practically proves, how- 

 ever, that the scoriae have been produced by the melting down of an 

 original clastic material which resembled in all its details the loess of 

 this region. The observed facts indicate, in brief, that the scoriae 

 are simply fused loess, melted under conditions which protected the 

 molten mass from oxidation. 



In this connection the relatively local distribution of the scoriae 

 near the coast is significant. Had the scoriae been transported from 

 the far West their size and number would naturally increase in 

 that direction, but the field observations show that the opposite is 

 the case. 



In seeking for a possible explanation of these phenomena and, in 

 particular, of the genesis of the scoriae, the writers have been forced 

 to adopt the following hypothesis as the only reasonable one in view 

 of the evidence at hand. This is presented, not as an established 

 theory, but only as a tentative hypothesis which needs further 

 verification but which accounts satisfactorily for the observed facts. 

 According to this hypothesis the loess formation was intruded by 

 igneous masses which melted down the adjacent loess and formed 

 the present black scoriae. These intrusions may have been sub- 

 marine or beneath the land area. In either case oxidation would 

 not have been serious, although a submarine extrusion might favor 

 less oxidation and more pronounced vesicular character of the 



