116 abstracts: petrology 



widespread volcanic activity of the explosive type during or just pre- 

 ceding the formation of the loess. 



The specimens of tierra cocida are composed, for the most part, 

 simply of loess fragments which have been indurated and reddened by 

 heat action, between 850° and 1050°. The loess and tierra cocida are 

 similar in general character and composition. 



The scoriae are not normal volcanic scoriae. They have been 

 produced by the melting down of an original clastic material (loess) 

 under conditions which protected the molten mass from oxidation. The 

 hypothesis is advanced that the loess formation was intruded by igneous 

 masses which melted down the adjacent loess and formed the present 

 black scoriae. The lack of oxidation of the scoriae and their abundance 

 in the field precludes the possibility that they were formed by the melt- 

 ing down of loess by bonfires or any type of fire in the open air. Pre- 

 historic man is not, therefore, responsible for their occurrence. 



F. E. W and C. N. F. 



PETROLOGY. — Study of a contact metamorphic ore-deposit. The 

 Dolores mine, at Matehuala, S. L. P. Mexico. J. E. Spurr, G. H. 

 Garrey, and Clarence N. Fenner. Economic Geology, 7: 

 444-484. 1912. 

 This is a study of an interesting jjroblem in applied geology, toward 

 which the laboratory contributed an exhaustive petrographic investi- 

 gation of the various types of rock which had been collected in the field. 

 The problem comprised the study and elucidation of the phenomena of 

 an unusual type of metamorphism and ore-deposition, associated with 

 and consequent upon the intrusion of a great mass of eruptive rock into 

 a series of sedimentary beds. In conjunction with the field-work a 

 collection of typical rock-specimens was sent to the laboratory. From 

 these the field relations which had been observed were confirmed and 

 supplementary information obtained. From the different sources of in- 

 formation thus made available it was possible to arrive at well-grounded 

 conclusions regarding the geological history of events, including the 

 sequence of mineral deposition, the nature of the circulating solutions, 

 and the relations existing between the metallic sulfides and the gangue 

 minerals. C. N. F. 



