ON THE NATURE OF IGNEOUS INTRUSIONS 179 
During the intrusion of a sheet of molten rock between 
horizontal strata, it is evident that the beds above the plane of 
intrusion must be lifted. The force which lifts the superior 
layers must be the dynamical energy of the intruded molten 
rock, aided perhaps by the steam generated when the highly 
heated magma came in contact with water. If intruded sheets 
were confined to the side of folds or to regions deformed in other 
ways, it might be suggested that the force which deformed the 
rocks tended to separate the strata, thus lessening the work that 
the intruded rock had to perform in order to make room for 
itself, but as intrusive sheets seem to be most common if not 
confined to regions where the receiving terrace is horizontal or 
was yet undisturbed at the time the intrusion took place, no such 
assistance can be claimed. That the receiving terrane should be 
practically horizontal at the time extensive intrusive sheets are 
formed, seems an essential condition, since folded and tilted 
strata are apt to be broken and faulted, and would thus furnish 
passages for the escape of the molten rock forced in among them 
under great pressure, and dikes and not sheets would result. 
Also, horizontally stratified beds would offer less resistance to 
the advance of an intruded sheet among them, than similar beds 
when folded, since the force necessary to split open the even 
grain of the horizontal beds, would be less than the force required 
to separate the contorted grain of folded beds. 
Another significant fact, as shown by observation, is that 
intruded sheets of wide extent at least, are composed of basaltic 
rocks, that is of the most easily fusible of igneous rocks. Rocks 
of which basalt may be taken as the type form highly fluid 
magmas when heated to 2000° or 2500° F., a temperature at 
which more siliceous rocks like rhyolite, are still solid or at 
most only viscous. Intruded sheets, therefore, seem to have 
been formed by the injection of highly fluid magmas under great 
pressure. Just as easily fusible basaltic lavas, when ejected from 
volcanoes, tend to spread widely over the surface, so highly fluid 
magmas, when forced between stratified beds, spread out in thin 
intruded sheets. As highly viscous magmas, when extruded at 
