104 PROCEEDINGS OP THE WASHINGTON MEETING 



One can hardly avoid the conclusion that the Mississippian went over the 

 arch ; and if the Mississippian was thoroughly baseleveled before the deposition 

 of the Pennsylvanian, it is hard to escape from the inference that the latter 

 must also have stretched across. 



Presented in abstract extemporaneously. 



Discussion 



Prof. Frank R. Van Horn : In Professor Miller's discussion of the faults, 

 with mention of the barite veins, he omitted to state that the veins contain 

 small amounts of galena and sphalerite. It is probable that the Lexington 

 veins are similar in origin of those of southern Illinois and western Kentucky, 

 in Caldwell and Crittenden counties, where the same mineral association was 

 found, except that fluorite is the gangue mineral instead of barite. In this 

 latter region the veins were worked at first for the lead, but lately for the 

 fluorite, which was used as a flux. 



MECHANICS OF INTRUSION OF THE BLACK HILLS PRECAMBRIAN GRANITE 



BY SIDNEY PAIGE 



(Abstract) 



A consideration of the Black Hills granite intrusion may throw light on the 

 mechanics of intrusion in general. 



Field relations indicate that the Black Hills granite came into its present 

 position in the main by physical distension of the invaded rock body, under 

 great load. The schists were deformed by the advance of the magma ; were 

 forced into closely appressed recumbent folds. The schistosity produced by 

 this folding lent itself to further injection by the granite through great num- 

 bers of parallel dikes and by intricate lit-par-lit intrusion. The harder rocks — 

 the quartzites — were distended and broken apart by the upward movement 

 and the magma flowed in between the blocks. 



It is believed the magma was intruded at a relatively low temperature ; that 

 it contained considerable water, and acted as a relatively mobile mass. 



The relation of dikes to schist layers, the fact that lit-par-lit injection tends 

 to neutralize chemical and physical differences between magma and invaded 

 rock, the probable low temperature of the granite, and the fact that positive 

 evidence of much assimilation is lacking — all support the belief that this 

 process was but a corollary of physical displacement, and not a primary process 

 by which the forward movement of the magma was accomplished. The possi- 

 bility that large blocks of roof have been engulfed in the magma, and that this 

 process was an important one, must remain an open question. 



Presented in abstract extemporaneously. 



Discussion 



Prof. Joseph Barrell: Mr. Paige has shown in his discussion of the field 

 relations of the Black Hills granite that they indicate intrusion in the main, 

 by physical distension of the invaded rock body, under great load. 



