ON THE NATURE OF IGNEOUS INTRUSIONS 183 
mines this behavior, but the principle which interests us most at 
present, is that different magmas under the operation of similar 
pressure tending to intrude them, would behave differently. The 
conditions that modify and limit the formation of laccolites have 
been fully discussed by Gilbert,’ and need not be reviewed at 
this time. 
Plutonic plugs.—I\n a previous article in this JOURNAL, already 
referred to, I have directed attention to the plug-like intrusions 
that occur in the Black Hills region. These intruded masses 
have been classed as laccolites by Crosby, and possibly the vari- 
ation they present from that type is not sufficient to necessitate 
a separate consideration. They occur in a region where the 
stratified rock into which they were forced are essentially hori- 
zontal, and where dikes and other evidences of fracture are 
absent, and where deep erosion has occurred. They are formed 
of refractory acid rock. The principal mineral constituent, accord- 
ing to Casswell, is always sanadine. 
The Black Hills of Dakota.—As pointed out in the essay 
referred to above, and as previously stated by Newton in his 
report on the Black Hills, the domes of sedimentary strata 
upheaved by the intrusion of plugs of igneous rock in the case 
of the Sun Dance hills, Inyan Kara, etc., are structurally iden- 
tical with the vast Black Hills dome. In each case previously 
horizontal strata were raised by a force acting from below verti- 
cally upward, so as to form a dome from the summit of which 
the layers dip away inall directions. The dome known as Little 
Sun Dance hill is unbroken by fractures and the summit of the 
plug of igneous rock inferred to exist beneath it, is not exposed. 
The absence of dikes in connection with the associated uplifts 
indicates that they, also, were not fractured as they rose, so as 
to allow the fluid rock beneath to escape. The absence of dikes 
indicates that the intrusion of the plugs took place at great 
depths, and that the upraised strata were under heavy pressure. 
The strata, after being upraised, were certainly of greater super- 
ficial extent than previously, when horizontal, and a stretching 
t Report on the geology of the Henry Mountains. 
