106 PROCEEDINGS OP THE WASHINGTON MEETING 



Mr. Charles E. Deckek : Having heard that an unconformity had been found 

 in the Precanibrian in the Black Hills, I should like to ask Mr. Paige about 

 the conglomerate he mentions in his paper. Is it a basal conglomerate, and 

 is there evidence of an unconformity beneath it? 



Brief remarks were made by Prof. E. A. Daly. 



Mr. Paige replied : I am gratified that both Professor Daly and Professor 

 Barrell agree that the Black Hills granite may have come to place in such a 

 manner as I have indicated. The idea that the variation in the mechanics of 

 igneous intrusion is a function of depth and temperature has long appealed to 

 me as reasonable ; that magmas and the rocks that they invade must act 

 differently at varying depths seems a logical deduction from what we know of 

 the earth's temperature and pressure gradient and from what we have observed 

 in the field. The idea, carried to its ultimate analysis, demands that there 

 should be a region at very great depths where, due to intricate injection and 

 prolonged impregnation by the magma, sediments will break down and lose 

 their identity as such, taking on all the characteristics of igneous rocks. 



PRECAMBRIAN STRUCTURE OF THE BLACK HILLS, SOUTH DAKOTA 

 BY SIDNEY PAIGE 



(Abstract) 



A study of the Precanibrian rocks of the Black Hills reveals a great series 

 of slates and schists, for the most part monotonously alike, striking in a north- 

 west direction, and having steep dips, generally, except in the extreme south- 

 west, to the east. At the south intrusions of granite break through the strata, 

 and around the principal mass of granite about Harney Peak a schistosity is 

 developed parallel with the granite contact. 



A closer study shows that the persistent eastward dips represent both schis- 

 tosity and bedding, the two for the most part parallel, and that the series is 

 compressed into a number of great folds which comprise innumerable minor 

 isoclinal folds. 



And. finally, a sufficient number of individual beds can be traced to locate 

 the position and nature of the greater axes of folding and to detect the pres- 

 ence of two important faults. 



The nature of the granite intrusion is such as to lead to the belief that the 

 mass exerted an important influence on the folding, and the position of one 

 of the faults was the determining factor in the localization of the great Home- 

 stake ore body. 



Presented in abstract extemporaneously. 



TITLES AND ABSTRACTS OF PAPERS PRESENTED BEFORE TILE AFTERNOON 

 SESSION" AND DISCUSSIONS THEREON 



Tbe Society convener! at 2 o'clock p. m., with President Coleman in 

 the chair. 



The Society proceeded immediately to the consideration of scientific 

 papers. 



