ON THE NATURE OF IGNEOUS INTRUSIONS 193 
result of such an intrusion is the generation of steam. In the 
case of the intrusion of a plutonic plug for example, steam may 
be generated and assist in raising the dome that is formed in the 
stratified beds above. When a plutonic plug on approaching the 
surface comes in contact with subterranean water, sufficient steam 
may be generated to blow away the dome above. Surface explo- 
sions sometimes remove large portions of volcanic mountains, as 
in the case of Krakatoa, Somma, Santorin, Barren Island, Cose- 
guina, Crater Lake, etc.; plutonic explosions may reasonably be 
supposed to remove domes of stratified rock (consisting either 
wholly of sedimentary beds, or made up in part of lava sheets), 
and form crater-like depressions termed crater-rings, calderas, 
etc.,as in the case of Coon Butte, Arizona; Lonas Lake, India; 
and still greater cavities, like those occupied by Lakes Bolsena 
and Bracciano, Italy, the former of which is circular and six and 
one-half miles in diameter, and the latter less regular, with a 
north and south diameter of ten and one-quarter miles, and a 
breadth of nine miles. 
The breaking of the steam bubbles that rise through the 
boiling lava in the crater of Vesuvius cause the summit of the 
mountain to tremble with miniature earthquake shocks. When 
the throat of the volcano becomes clogged, steam generated 
within breaks through the obstruction or rends the mountain 
with such violence that the region for miles around is severely 
shaken. Subterranean intrusions of molten rock on coming in 
contact with water, may reasonably be supposed to cause similar 
steam explosions of which the only surface manifestations would 
be earthquake shocks. 
Since my essay on the plutonic plugs of the Black Hills 
region was published, my attention has been directed to the 
fact that the rocks, of which some of the plugs are composed, 
have been recently studied in the light of more modern petro- 
graphical methods than were employed by Caswell. L.V. Pirsson, 
states, in the Am. Jour. Sci., Vol. XLVII., 1894, pp. 341-346, that 
the rock forming the cores of Mato Teepee and the Little 
