96 BUREAU OF A.MERTCAN ETHNOLOGY [bull. 52 



scoriae than a subaerial extrusion. In view of the exceedingly large 

 amount of volcanic glass and minerals of igneous origin in the loess 

 of this region, the assumption that volcanic action has been directly 

 responsible for the formation of these scoriae and also of the tierra 

 cocida, in the manner suggested above, is not unreasonable. It 

 is surprising, however, that the present collection does not contain 

 any specimens of normal lava from either Necochea or Miramar. 

 This may possibly be due to the fact that the collection is a small 

 one and probably does not include all the rock types which occur in 

 these locaUties; or it may be that the volcanic extrusion was of the 

 explosive type, whereby the lava (possibly siliceous in character and 

 largely glassy) was shattered and reduced to dust, which fell to the 

 surface as volcanic ash and now constitutes an integral part of the 

 loess formation. Under these conditions the cooler, viscous, melted 

 loess fragments would remain intact and be ejected as scoriae and 

 resist attrition and breaking down more effectively than the shat- 

 tered volcanic lava. 



Similar contact phenomena between lava intrusions and adjacent 

 rhyolite pumice breccia have been observed in Nevada by Professor 

 Iddings.^ There basaltic lava was plainly visible and the mode of 

 formation of the contact scoriae was obvious. The broken mineral 

 fragments occurred embedded in the glassy base which, in some of 

 the sections, still showed the brecciated character of the original 

 material. These relations are clearly stated in the following para- 

 graph from the general description of the rocks from this region by 

 Professor Iddings in the monograph cited above (p. 183): 



"Thin section 200 is the most interesting of all the alteration 

 products, on account of its undoubted relations to the basalt and its 

 higher degree of metamorphism ; it is traceable directly to the same 

 deposit of pumice as 199, and lies in apparently undisturbed layers 

 directly over basalt, which did not in this instance reach the surface, 

 but thoroughly altered the overlying pumice, breaking through it 

 lower down the slope. In thin section it is a whitish gray, fine- 

 grained breccia of about the same grain as 199. Under the micro- 

 scope the porphyritical crystals are seen to be angular fragments of 

 quartz, sanidine, and plagioclase of the same size and abundance as 

 those in the last-named section; pyroxene, however, is wanting and 

 onl}^ a little biotite is present, besides a single grain of garnet. The 

 groundmass has retained its brecciated character, though the pumice 

 fragments have lost their original form and appear to merge into one 

 another; but the degree of crj^stallization is far more advanced, 

 hardly any portion of it being without influence on polarized light. 



1 See Arnold Hague, Geology of the Eureka District, Nevada. U. S. Geological Survey Monographs, 

 No. 20, 381-385, 1892. 



