182 NU SWCAUBIE (Ox IRI SSI BING 
Utah, Colorado and Arizona, Cross? states that the rocks com- 
posing these intrusive bodies all belong to a well marked struc- 
tural type and present but slight variations in mineralogical com- 
position. Plagioclase is the predominant mineral of all the 
rocks — excepting the quartz-porphyry—and the uniform appear- 
ance of its stout white crystals gives character to the whole series. 
Which of the feldspars composing the group known as plagioclase, 
are most common in these rocks 1s not clearly stated, but.in a 
table of nineteen analyses, the percentage of silica ranges from 
56.62 to 73.50, and of alumina 14.87 to 18.00. The rocks are 
therefore highly siliceous, and, I think I am justified in conclud- 
ing, highly refractory, in comparison with the basaltic rocks of 
which widely extended intrusive sheets are composed. 
It appears, therefore, that the nature of an intruded magma, 
whether highly fluid or viscous, has much to do with the form in 
which it solidifies. The degree of fusion depends, of course, on 
the amount of heat. Even the most refractory rocks, when 
sufficiently heated, will become highly fluid, under normal sur- 
face pressure. Since the) intenon heat or thes earth) anereases 
with depth, refractory rocks would remain unmelted, or perhaps 
be partially fused and viscous, at a depth where the most easily 
fused igneous rocks would be highly fluid. This would lead us 
to expect also that laccolites would be formed deep in the 
earth’s crust. This inference can be checked by observation. 
Evidently, if laccolites are now exposed at the earth’s surface, 
the amount of erosion that has taken place in the disturbed 
beds above them will show how deeply they were originally 
buried. In the case of the Henry Mountains, Gilbert shows that 
the domes of stratified beds removed were possibly 7700 feet 
thick. 
The relative specific gravity of intruded magmas may also be 
expected to influence their behavior, and especially the height 
to which they would rise under a given pressure, but this seems 
to bea minor feature of the problem. The force with which 
magmas are injected is, of course, the main cause which deter- 
*U.S. Geological Survey. Fourteenth Annual Report, pp. 224, 228. 
