546 GEOTECTONIC (STRUCTURAL) GEOLOGY, [Boor IV. 
abundant lies about the base of the Carboniferous Limestone series. 
Along that horizon they rise to the surface for many miles, sometimes 
ascending or descending in geological position, and breaking here 
and there abruptly across the strata.’ There can be little doubt that 
they have actually melted down some parts of the stratified rocks, - 
particularly the limestone. Considerable petrographical differences 
occur among them, which may perhaps be in some measure due to 
the incorporation of such extraneous material into their mass. Gaps 
occur where these intrusive rocks do not rise to the surface, but as_ 
they resume their position again not far off, it may be presumed that 
_ they are really connected under these blank intervals. 
Mr. G. K. Gilbert has described, under the name of “ laccolite,” 
a structure in the Henry Mountains in Southern Utah, which is 
probably not uncommon in denuded volcanic districts. Large bosses 
of trachytic lava have risen from beneath, but instead of finding their 
way to the surface, have spread out laterally and pushed up the over- 
lying strata into a dome-shaped elevation. Here and there smaller 
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Fic. 273.—IDEAL SECTION OF THREE ‘ LACCOLITES,” AFTER GILBERT. 
sheets proceeding from the main masses have been forced between 
the beds, or veins have been injected into fissures, and the overlying 
and contiguous strata have been considerably metamorphosed.’ 
Effects on Contiguous Rocks.—Many intrusive bosses have 
greatly affected the texture, and even the mineralogical composition 
of the rocks through which they have been erupted. The amount and 
nature of the change produced vary with the character and bulk of 
the eruptive mass as well as with the susceptibility of the surrounding 
materials to alteration. Diorite, diabase, melaphyre, basalt, felsite, 
and other eruptive rocks are not infrequently accompanied by very 
considerable metamorphism of the adjacent strata. ‘These phenomena 
are manifested also by intrusive sheets, dykes, veins, and necks. They 
belong to the series of changes embraced under the head of contact 
metamorphism, and are grouped together for description in the next 
Part (p. 572). 
1 Trans. Itoy. Soc, Edin. xxix. p. 476. 
* Geology of the Henry Mountains, U.S. Geog. and Geol. Survey, Washington, 1877. 
The same structure was figured and described upwards of forty years ago by C, Maclaren, 
“ Geol, of Fife and Lothian,” 1839, pp. 100, 101. 

