180 USIATEIE, (Cs, LMOSSISIEIE 
the surface, flow sluggishly and frequently come to rest in thick 
masses even on steep slopes, it is to be inferred that if such 
magmas were intruded in a manner similar to that by which 
sheets of basalt are spread between sedimentary strata, they 
would expand much less widely. This difference in freedom of 
flow between magmas that are easily fusible and those that are 
refractory, appears to be one of the conditions which determines 
whether a body of highly heated rock intruded among horizon- 
tally stratified beds, shall spread widely and form a sheet, or be 
restricted in its lateral expansion and cause a more local uplift 
of the strata above it. 
Another condition which would influence the behavior of an 
intruded magma, is the depth in the earth’s crust at which the 
intrusion occurs. Since the rocks above the intruded magma have 
to be lifted, the higher in the series the intrusion occurs, the less 
the weight of the rocks above, and the greater the amount of 
energy available for lateral expansion. If variations in the specific 
gravity of strata are not considered, the conditions favoring the 
formation of intruded sheets, increase as the magma approaches 
the surface, until the tendency of the lifted strata to fracture deter- 
mines a limit. We should expect, therefore, that intruded sheets 
would be most numerous in the upper portion of the earth’s crust. 
The correctness of this inference can be tested to some extent by 
observation. The date at which many intrusive sheets were 
formed, however, and the amount of subsequent erosion that has 
taken place, are frequently difficult to determine and space will not 
permit of the introduction of evidence in this connection. The 
fact that the edges of intruded sheets are frequently exposed in 
canyon walls in regions of topographic youth, certainly favors the 
conclusion that widely extended intrusive sheets are compara- 
tively superficial phenomena. 
In order to bring the ideas which I wish to present to a focus, 
let us consider other phases of igneous intrusion. 
Laccolites—The difference between a widely extended intru- 
sive sheet and a local cistern-like injection of molten rock, or a 
laccolite, so far as the shape of the intruded mass is concerned, 
