CLASSIFICATION OF IGNEOUS INTRUSIVE BODIES 497 



Plug. — Russell has described as "plutonic plugs" certain intru- 

 sions occurring in the Black Hills of Dakota. They "are composed 

 of igneous matter forced into sedimentary strata and have a plug-like 

 form." 1 He continues: "How the stratified beds below the domes 

 that covered the plugs were displaced, or perhaps fused, so as to 

 furnish room for the passage of the intruded material, is not clear." 

 Again: "None of the plutonic plugs examined by me are associated 

 with dikes or faults." 2 "They occur in a region where the stratified 

 rock into which they were forced are essentially horizontal." 3 



Since neither the form nor the method of intrusion is clearly 

 indicated, it is difficult to classify "plugs" in Russell's sense. It is 



Fig. 5 



to be noted that Jaggar 4 and Iddings 5 interpret some of Russell's 

 original types as true laccoliths. Russell's statement does not make 

 clear the distinction between "plugs" and stocks. The name "plug" 

 has been rather commonly used as alternative with the magmatic 

 filling of a volcanic vent. 6 For these various reasons, "plugs" will 

 not be included in the proposed classification of this paper. 



Bysmaliths. — Allied to "plugs" in Russell's sense is the "bysmal- 

 ith" of Iddings, described as an injected body filling a "more or less 

 circular cone or cylinder of strata, having the form of a plug, which 

 might be driven out at the surface of the earth, or might terminate 

 in a dome of strata resembling the dome over a laccolith." 7 The 

 downward termination of the original type bysmalith (Mt. Holmes) 

 is found in a hypothetical Archean floor on which the porphyry of 

 the bysmalith rests. See illustrations. 8 



1 Journal of Geology, Vol. IV (1896), p. 25. 3 Ibid., p. 183. 



2 Ibid., p. 42. 4 Op. cit., p. 287. 



5 Journal of Geology, Vol. VI (1898), p. 706. 



6 Recently by Merrill, op. cit., p. 51, and by Chamberlin and Salisbury, op. cit., 

 p. 476. 



7 Monograph No. 32, Part 2, U. S. Geological Survey (1899), p. 16. 



8 Iddings, op. cit., p. 16; and Journal of Geology, Vol. VI (1898), p. 708. 



