ABSTRACTS OF PAPERS 129 



EVIDENCES OF ASSIMILATION DURING THE KATMAI ERUPTION OF 1912 



BY C. N. FENNER 



(Abstract) 



111 a paper published in November, 1920, evidence was cited for the view 

 that during the eruption of Katmai the collapse of the walls of the former 

 crater and their assimilation by tbc new magma were essential features of 

 the process that resulted in the formation of the present huge crater-pit. 

 Inasmuch as the properties of the magma implied by this conception are of 

 much theoretical interest, it has seemed desirable to look for further evi- 

 dences of assimilation. Considerable time has been given, therefore, to a 

 study of samples (collected in the field) of the successive strata of ejected 

 material. Two methods of investigation have been employed : First, the use 

 of a binocular magnifier as an aid in sorting out the different materials of 

 which the ejecta are composed, by which means quantitative expressions for 

 the progressive change in the make-up of the material have been obtained ; 

 and, second, a series of chemical analyses has been made, and the linear 

 variation of composition as assimilation progressed which the theory requires 

 has been found to hold true in a very marked manner. 



It is believed that this evidence gives strong confirmation to the idea of 

 assimilation on a large scale. Conclusions regarding certain properties of the 

 magma may be derived therefrom. 



Presented without notes, with lantern-slide illustrations. 

 Vice-President Keith withdrew and President Kemp took the chair. 



TRIDYMITE-ORTBOCLASE ROCK, A NEW METAMORPHIC ROCK TYPE FROM 

 IMPERIAL COUNTY, CALIFORNIA 



BY AUSTIN F. ROGERS 



(Abstract) 



A banded, porous tridymite-orthoclase rock occurring in a small dome- 

 shaped extrusion is proved by both field observation and microscopic examina- 

 tion to have been formed from a typical rhyolitic obsidian. Intermediate 

 stages in which cristobalite occurs can be recognized. The cristobalite occurs 

 in cavities of hollow spherulites, and also intergrown with orthoclase in solid 

 spherulites. The final stage, which consists of tridymite and orthoclase, is 

 considered to be a metamorphic rock ; the metamorphism was brought about 

 by hot gases. 



Presented from notes, with lantern -slide illustrations. 

 Discussed by A. C. Lane. 



Discussion 



Professor Lane : I would call attention to the fact that in general aporhyo- 

 lites and felsites would have to be classed as metamorphic if this rock is so 

 classed. Although etymologically correct, I would hesitate so to extend a 

 term which has so generally been confined to rocks which have recrystallized 

 under pressure to which they have yielded. 



IX — Bull. Geol. Soc. Am., Vol. a: 1 ,, 1021 



