BS MEME MES: 
A LACCOLITH as defined by Gilbert” is a body of igneous rock 
which has forced itself by intrusion, in a molten condition, 
between strata of sedimentary rocks in such a manner as to have 
lifted the overlying strata in a dome-shaped arch above it. The 
arching strata may be stretched and cracked to variable degrees 
and the molten magma may penetrate them to a greater or less 
extent according to the character of the rocks and the amount 
of cracking. A symmetrical dome may be the ideal form of a 
laccolith, but as Cross? has shown is rarely assumed because of 
the many modifying conditions attending the process. The 
principal ones are: a position of the plane of fracture along 
which intrusion takes place oblique to the bedding of the strata ; 
lines of structural weakness in the strata; the presence of earlier 
intrusive bodies; the lack of coherence and of pronounced bed- 
ding in the strata invaded. Gilbert’s use of the term laccolith 
embraced all lenticular bodies of igneous, intrusive rock occur- 
ring in stratified sedimentary rocks. 
A laccolith is to be distinguished from an intrusive sheet of 
igneous rock, which also has been in most cases intruded 
between strata that were more or less horizontal at the time and 
required to be lifted by the force within the molten magma. The 
difference lies in the thickening of the igneous body into a more 
or less lenticular mass in the case of the laccolith, and in its 
retaining an almost uniform thickness in that of a sheet. In 
both cases the act of lifting the superincumbent strata must 
have been due to the same kind of force, namely, that exerted 
by a liquid under pressure upon the sides of a containing reser- 
GILBERT, G. K., Report on the geology of the Henry Mountains, U. S. Geog. 
and Geol. Surv. of the Rocky Mountain region, J. W. PowELL in charge. Washing- 
ton, 1877. 
2Cross, W., The Laccolithic Mountain Groups of Colorado, Utah, and Arizona. 
r4th Ann. Rep. of the Director of the U. S. Geol. Surv., pp. 236. Washington, 1895. 
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