128 PROCEEDINGS OF THE BOSTON MEETING 



with a greenish tinge. Feldspars which occur in grains 7 millimeters or less 

 in diameter are generally more abundant than the other minerals, which in- 

 clude muscovite, magnetite, pyrite, zircon, apatite, chlorite, calcite, and kaolin. 

 Much of the tuff, however, consists of devitrified and silicified volcanic glass, 

 of which a large part is ordinary volcanic ash and represents fragments of 

 the walls of bubbles, though some of the larger fragments of glass are fluidal. 

 The original rock from which the tuff was derived was probably near a rhyo- 

 lite or quartz latite. 



None of the tuff beds have yielded fossils, so their assignment to the Missis- 

 sippian series is based upon the relations of the Stanley shale to overlying 

 and underlying rocks, whose ages have been determined -by fossils. 



The conclusion is that the tuff had a southern source. This source was 

 probably on or near a large land area that existed in Louisiana and eastern 

 Texas not only during the Mississippian epoch, but during the greater part of 

 the Paleozoic era. 



Eead in full from manuscript. 



Discussion 



Dr. Sidney Powers : Evidence of the existence of a land-mass in east-cen- 

 tral Texas in Paleozoic time is found in well records. Precambrian schist and 

 granite are found directly beneath the cretaceous beds south and east of the 

 Llano Burnet region. Ordovician limestones overlie the Precambrian granite 

 in the Red River district in Cooke, Montague, and Clay counties, and they are 

 in turn overlain directly by Pennsylvanian sediments. 



Eemarks were also made by Prof. Charles Schuchert, with reply by 

 the author. 



TYPES OF ROCKY MOUNTAIN STRUCTURE IN SOUTHEASTERN IDAHO 

 BY GEORGE R. MANSFIELD 



i Abstract) 



The paper gives additional data regarding the Bannock overthrust, described 

 by R. W. Richards and the author in 1912, and discusses briefly a number of 

 types of mountain structure recognized in seven quadrangles (part of the west- 

 ern phosphate field) in southeast Idaho. The types include elongate and 

 curved zigzag folds; eroded fan folds, some of which are broken by faults; 

 transverse folds, one of which has developed into a transverse thrust-fault; 

 horst and graben structure, strikingly cross-faulted; and noteworthy uncon- 

 formities. Mention is made of the supposed conditions attending the defor- 

 mation. 



Presented without manuscript, with lantern-slide illustrations. 



Discussion 



Dr. G. W. Stose: The author states that the great overthrust fault-planes 

 were flat and not deep, which is in accord with conclusions I have reached in 

 regard to such faults. The question is raised whether the normal faulting, 

 which cuts and offsets the thrust blocks, was simply due to a readjustment in 



