88 BUREAU OP AMERICAN ETHNOLOGY fBULL. 52 



Thermal evidence. — On heating these loess specimens to tempera- 

 tures between 850° and 1,050° distinct sintering occurs. The brown, 

 earthy material assumes a brick-red color and shrinks to a compact, 

 indurated mass, resembling brick in all its properties. Between 

 1,050° and 1,150° it melts to a thick, viscous glass, in which the 

 mineral fragments appear to be set as in a thick paste. On con- 

 tinued heating the mineral fragments are slowly dissolved by the 

 molten glass. By lowering the temperature 100° or more microlites 

 ciystallize out, whose optical properties agree with those of pyroxene, 

 but of a variety different from that of the pyroxene fragments in 

 the original loess. Occasionally minute laths of plagioclase were also 

 precipitated, together with the pyroxene microlites. The glass 

 thus formed is streaky and evidently variable in its composition. 

 Its refractive index varies rapidly from about 1.51 to above 1.56 in 

 some of the specimens. On heating the loess to the temperature at 

 which melting first occurs the glass formed is practicall}^ colorless 

 and contains many opaque particles of iron oxide. On raising the 

 temperature the magma dissolves these particles and the resulting 

 brown-colored glass has a notably higher refractive index; the more 

 pronounced the color of the glass, the higher its refractive index. 



In the thermal experiments with the loess specimens it was found 

 that the mode of treatment and size of the charge had great effect 

 on the resulting product. Small, loose charges of powdered material 

 were necessarily exposed during heating to the oxiziding effect of the 

 air and the iron present was converted largely into the ferric state, 

 with tlie result that the product was colored dark-red. With heavy 

 charges of large lumps of the loess, the air found less ready access to 

 all parts of the material and the outer portions only of the lumps 

 were thoroughly oxidized, the centers of the lumps being dark-gray 

 in color and resembling the scoria in appearance. (PL 7.) In con- 

 ducting thermal experiments of this kind in imitation of natural 

 processes, it is highly essential that the physical conditions of experi- 

 ments be as nearly like those which obtain in nature as it is possible 

 to make them, otherwise totally different products may result and the 

 conclusions deduced therefrom be in error to that extent. 



TIERRA COCIDA 



Petrographic microscopic features. — Under this title three distinct 

 types of rock are included: 



(1) Indurated, brick-red earths, which, under the microscope, 

 resemble the ordinary loess specimens except for the red particles 

 which have evidently resulted from the heating of the argillaceous 

 material. The mineral composition of the tierras cocidas both with 

 respect to kind and size is practically identical with that of the 



