18 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



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. By this mode of 

 intrusion the vertical dimension of the intruded mass becomes still greater 

 as compared with the lateral dimensions, so that the shape is more that of a 

 plug or core. Such an intruded plug of igneous rock may be termed a 

 bysmalith (0v(?/u a —plug, Az0off=stone). We have, then, transition from 

 a flat, thin, intrusive sheet to a laccolith with lenticular form, and from 

 this to a bysmalith with much greater depth and considerable vertical 

 displacement. 



Examples of bysmaliths are not common as yet. Russell 1 has called 

 attention to what he considers volcanic plugs in the region of the Black 

 Hills of Dakota, and has suggested their recognition as types of intrusion 

 different from normal laccoliths. A sharp discrimination of the two types 

 may not always be possible,, since they grade into each other, as in Mount 

 Marcellina. In the intrusive bodies of Mount Holmes and the Indian 

 Creek laccolith the contrast is sufficiently marked and the two types are 

 well illustrated. Nearly two-thirds of the circumference of the Holmes 

 mass is exposed as a nearly vertical plane of contact crossing almost hori- 

 zontal strata. The western boundary is ag'ahist gneiss and along a fault 

 plane of considerable magnitude, which probably acted as the conduit 

 through which the magma was forced. There is no means of knowing 

 what may be the shape of the bottom of this bysmalith. It is possible 

 that it may have risen through the fault fissure until it encountered the 

 sedimentary strata resting upon the gneiss, with its inclosed laccolith. It 

 may have spread laterally along shaly strata near the gneiss and beneath 

 the laccolith; then its movement laterally may have been checked, for 

 the pressure upward became sufficient to rupture the strata and laccolith 

 and to force a mass of these rocks covering an area of over 5 square miles 

 up more than '2,000 feet — probably more than twice that height. 



The areal relation of the Mount Holmes bysmalith to the surrounding 

 terranes is shown on the geological map, PI. X. The vertical relations are 

 shown in the profile sections of the Gallatin Mountains, PI. Ill, fig. 3, and 

 PI. IX, fig. 4, and in PI. V, figs. 1, 2, 3, which are profile sections through 

 Echo Peak and Three River Peak, and through The Dome and the peak 



'Russell, I. C, Jour. Geol., Vol. IV, 1896, p. 23. 



