BL-NCK HILLS GEOLOGY. 239 



fined his attention to several of the more prominent igneous up- 

 hfts that are far removed from the axis of the hills. Of these 

 he studied four : Sundance hill, Mato Tepee, Little Sundance 

 hill and the Little Missouri buttes. For the others he referred 

 to the accurate descriptions of Newton. Like Newton, he was 

 impressed by the local nature of the disturbances and by the 

 uplifted strata around them. 



He further calls attention to the entire absence of dikes and 

 auxiliary intrusives in their neighborhood, and accounts for 

 their form by the assumption that they were " plutonic plugs " 

 injected into overlying strata from below with a force sufficient 

 to perforate portions of the sediments, but still buried deep be- 

 low the surface. His theory, as that of Newton, implies a 

 viscidity such that the formation of fluid intrusions like sheets 

 and dikes was an impossibility, and differs from the first theory 

 only in regard to the large amount of superincumbent strata. 

 It is in part owing to the great pressure exerted on the intrusion 

 by this overlying rock that he attributes the viscidity, impossi- 

 ble in a surface eruption. Li a second paper he discusses the 

 general nature of intrusions and elaborates his theory of " plu- 

 tonic plugs " from phenomena observed in the hills. The paper 

 is too long for quotation, and for a complete review the reader 

 is referred to the same, but the points with which we are mainly 

 concerned are these : 



The distance to which an intrusion will extend laterally is depend- 

 ent largely on the consistency of the intruded rock ; if fluid it will 

 extend to great distance, as the Palisades diabase ; if slightly viscid 

 it will produce a less extended upheaval and will from a laccolite ; 

 and, finally, if very viscid it will form an extremely local upheaval, 

 as the "plutonic plugs" of the Black Hills. Such viscidity is a 

 function : First of the chemical composition of the rock, acid rocks 

 being more viscid than basic ; and second, of the pressure exerted by 

 the depth of burial, the more deeply buried being the least fluid. 

 Newton has shown that the rocks forming the peaks discussed are 

 acid rocks, and also that the strata which formerly covered them 

 were of a probable depth of 4 000 feet. Both conditions for the 

 existence of " plutonic plugs " are then fulfilled. The peaks studied 

 are of this character, and from the writings of Newton the others 

 seem to be. 



