ON THE NATURE OF IGNEOUS INTRUSIONS 181 
is mainly in the degree of lateral expansion. In what may be 
termed their genetic features, these two classes of intrusions are 
essentially similar. In each case molten rock rises from below 
into stratified beds, probably through fractures, and on reaching 
the upper limit of the fractures lateral expansion takes place and 
the strata above are lifted. The main conditions, as we have 
seen, which control the extent of the lateral expansion, besides 
the propelling force and the occurrence of fractures or other 
openings through which the molten rocks rise, are the fluidity of 
the magmas, and the depth in the earth’s crust at which they 
reach the upper limit of the passage-way through which they 
came, and tend to spread horizontally. Under the same condi- 
tions respecting temperature and pressure, we should expect 
refractory magmas to form laccolites, while more easily fusible 
rock would be more apt to spread out in sheets.1 We should 
expect laccolite, therefore, to be composed of less easily fusible 
rocks than are found in widely extended sheets. Again we may 
test inference by observation. It is usually conceded that the 
amount of silica in a rock determines its degree of fusibility. 
Acid rocks, as a rule, are more refractory than basic rocks. Dana 
has qualified this conclusion, however, by showing that it is the 
fusibility of the chief constituent minerals in a lava which deter- 
mines its mobility. He says: ‘Trachyte and rhyolite are the 
least fusible of igneous rocks, because the constituent feldspar, 
orthoclase, is the least fusible of the feldspars; and basalt or 
dolerite is one of the most fusible, because the feldspar present, 
labradorite, is of easy fusibility, and it is combined in the rock 
with still more fusible augite and pyroxene.” 
In a recently published report on the laccolitic mountains of 
*Intruded sheets do occur in connection with the laccolites, and are composed of 
off-shoots of the same material. Sometimes these sheets, as stated by Cross, extend 
four or five miles from the main intrusion. As shown in sections of Henry Mountains, 
given by Gilbert, the associated dikes and sheets are in the disturbed strata above and 
immediately about the main body of the laccolites. The opening of the deformed strata 
by the disturbance caused by the principal intrusion appears to have had much to do 
with the origin of the secondary phenomena. It does not seem to me that the com- 
paratively small sheets originating in this manner furnish an exception to the general i- 
zation suggested above. 
