WRIGHT-FEJNNBR] PETEOGKAPHIC STUDY 93 



vesicular and even pumiceous but no evidence was obtained to sug- 

 gest even a relation between this rock and the scoriae from Miramar 

 and Necochea. 



General Conclusions and Summary 



In the foregoing })ages the attempt has been made to present the 

 data of observation and experiment on the collection of rocks sub- 

 mitted to the writers for determination. Throughout the investiga- 

 tion but little attention has been given to the works of others on the 

 same general problem. The present writers have not personall}'- 

 studied tlie rocks in the field and are not in a position, therefore, to 

 discuss the more general problem of which the rock problem is only 

 a part. In the descriptions attention has accordingly been directed 

 to the facts of observation rather than to the possible theories of 

 genesis which, without a definite basis of fact, become simply matters 

 of opinion. 



The most important facts noted during the present investigation 

 are: 



(1) The loess consists in large measure of volcanic and eruptive 

 material. Siliceous^ volcanic glass is present in practically every 

 specimen and may become so abundant that it constitutes 90 per 

 cent of the whole. The minerals present are remarkably fresh and 

 unaltered. The amount of argillaceous material present is relatively 

 small in most of the specimens. These facts may be considered 

 indicative of tremendous and widespread volcanic activity of the 

 explosive type during or just preceding the formation of the loess. 

 The variation in composition of the loess may be due in part to the 

 effect of wind action on original tuffaceous deposits, the wind blowing 

 the lighter matter farther away from the source, and thus producing 

 a rough separation of the components in some instances. That 

 volcanic action was widespread and not confined to one vent is 

 evident from the enormous quantity of volcanic material in the loess. 



(2) The specimens of tierra cocida are, for the most part, com- 

 posed simply of loess fragments which have been indurated and 

 reddened by heat action between 850° and 1,050°. Except for the 

 red particles which have resulted from the oxidation of the iron in the 

 argillaceous material of the loess, the loess and most of the tierra 

 cocida are identical in general character and composition. Analyses 

 of both are closely similar and, except for the higli water content, 

 might be mistaken for analyses of igneous rocks. This is well shown 

 by specimens 263717, 263718, 263729 from Alvear, whose micro- 

 scopic features, behavior on heating, sodium cliloride content, and 

 the chemical analyses of 263717 and 263729 are as nearly alike as can 

 possibly be expected from such material and prove their original 



