486 PEOCEEDINGS: GEOLOGICAL SOCIETY 



gold deposits are confined to the basal member of the geologic column. 

 The gold occurs both in the native state, and intergrown with pyrite. 

 The gangue is dominantly siliceous, with calcite occasionally present. 

 The gold ores are more often in well-defined veins than are the copper 

 deposits. 



Discussion: Sidney Paige inquired whether acid solutions might 

 be capable of depositing the calcite of the ore-bodies, as might be 

 inferred under the supposition that the deposits were due to atmos- 

 pheric waters. Mertie thought it unlikely. Schrader inquired 

 regarding the thickness of the basal formation. Mertie explained 

 that folding made it difficult to compute this, but thought that it was 

 probably about equal in thickness to the Nikolai greenstone, or 6,500 

 feet. 



Chase Palmer: The silver preci-pitating capacity of certain arsenides 

 as an index of their constitution. The speaker described a number of 

 experiments he had made on certain metallic arsenides, in which they 

 had been subjected to the action of silver solutions. Their silver- 

 precipitating capacity was believed to afford an indication of the valence 

 of the iron present. 



Discussed by Sidney Paige. 



G. W. Stose: The mechanics of a cross fault in the Northern Appa- 

 lachians. Cross faults in the Appalachians are generally of two kinds: 

 (1) Faults in which the thrust plane is so flat that, when slightly folded, 

 erosion produces deep reentrants across the strike; (2) shear faults 

 accompanying thrust faults, resulting froiii differential displacement. 

 The fault here described is of the latter class. 



South Mountain is broken by a cross fault in southern Pennsylvania. 

 The rocks aff'ected are pre-Cambrian lavas, Cambrian sandstones, 

 quartzites, conglomerates, and shales, and Cambro-Ordovician lime- 

 stones. The fault passes under cover of the Triassic sediments east of 

 the mountain. Adjacent to the cross fault the formations have been 

 offset a mile or more. Drag of the beds along the fault is conspicuously 

 shown on both sides. A transverse depression across the mountain 

 eroded along the crushed zone of faulting conceals the exposures, so 

 that fault brecciation was not observed, but the adjacent metarhyolite 

 is crushed and intensely jointed. Two major anticlines and the en- 

 closing syncline are the chief structures in the area. Overturned folds 

 and a prevailing southeastward-dipping schistosity show that the 

 direction of thrust and pressure at the level here studied is from the 

 southeast. Therefore the eastern of the two anticlines was the first 

 to rise, and the oldest pre-Cambrian rocks are brought up along its 

 axis. The western anticline did not rise so high and is still capped by 

 the massive Cambrian sandstones which plunge south and pass under 

 the limestone at the cross fault. North of the cross fault this western 

 massive anticline of sandstone blocked the way to further westward 

 movement and the rocks in the syncline between the two anticlines were 

 intensely folded and faulted. South of the cross fault, however, the 

 Cambrian sandstone lay so low that westward movement was not 



