28 
LACCOLITHIC MOUNTAINS 
Type-form of Laccoliths 
Ideal Cross-section of Henry Mountains. According to Gil¬ 
bert laccoliths, sheets and dikes are genetically one. “Between 
the sheet and laccolith there is complete gradation. The lacco¬ 
lith is a greatly thickened sheet and the sheet is a broad, thin, at¬ 
tenuated laccolith.’' The familiar diagrammatic representation of 
a laccolith (figure 1) indicates a simple bulge of strata by out- 
Fig. 1. Ideal Form of Laccolith. (After Gilbert.) 
wardly forced magma which from some cause or other does not, 
as in normal volcanic eruptions, reach the surface of the ground, 
but insinuates itself between rock beds at an horizon some distance 
from sky. 
It is the mechanical possibility such as the ideal conception 
which Gilbert conveys that is so sharply challenged by European 
geologists, notably Suess,^^ Neumayer,^^ Reyer,^^ and Geikie.^'^ 
Still other features seem yet quite inexplicable and still are ques¬ 
tioned by many American investigators. In view of Neumayer’s 
categoric statement that the Henry Mountains intrusion as ex¬ 
plained by Gilbert “borders almost on the impossible and incred- 
itable” the query arises as to what conditions, if any, exist by 
which laccolithic intrusion becomes a potential mechanic reality. 
In the light of subsequent and more complete observation the 
simple blister form of magmatic body probably never does actually 
obtain. As a geological phenomenon it now seems hardly more 
10 Geology of the Henry Mountains, p. 20, 1879. 
11 Das Antlitz der Erde, I Bd., p. 195, 1885. 
12 Erdgeschichte, I Bd., p. 180, 1887. 
13 Theoretische Geologie, p. 135, 1888. 
14 Text-book of Geology, 4th Ed., Vol. II, p. 736, 1903. 
