CHAP. XLIV 
HISTORY OF THE GABBRO INTRUSIONS 
363 
masses of trachyte have been injected between sedimentary strata belonging 
to the Jura-Triassic and Cretaceous systems. These masses, thirty-six in 
number, have consolidated in dome-shaped bodies, termed by Mr. Gilbert 
“ laccolites,” which have arched up the overlying strata, sending sheets, veins 
and dykes into them, and producing in them the phenomena of contact 
metamorphism. There is no proof that any of these protrusions communi- 
cated with the surface, and there is positive evidence that most if not all of 
them did not. The progress of denudation has laid bare the inner structure 
of this remarkable type of hill, and yet lias left records of every stage 
in its sculpture. I11 one place are seen only arching strata, the pro- 
cess of erosion not having yet cut down through the dome of stratified rocks 
into the trachyte that was the cause of their uprise. I11 another place, a 
few dykes pierce the arch ; in a third, where a greater depth has been bared 
away, a network of dykes and sheets is revealed ; in a fourth, the surface 
of the underlying “ laccolite ” is exposed ; in a fifth, the laccolite, long 
uncovered, has been carved into picturesque contours by the weather, and 
its original form is more or less destroyed . 1 
The gabbro “ laccolites ” of the West of Scotland belong to an older 
geological period than those of Utah, and have, therefore, been longer subject 
to the processes of denudation. They have been enormously eroded. The 
overlying cover of basalt has been stripped off from them, though from the 
escarpments beyond them it is not difficult in imagination to restore it. In 
Bum it has been so completely removed, that only a few fragments remain 
at some distance from the core of gabbro, which now stands isolated. In 
Ardnamurchan, and still more in Skye, the surrounding plateau of basalt 
remains in contact with the gabbro bosses. But in Mull, where the plateau- 
basalts reach now, and perhaps attained originally a greater thickness than 
anywhere else, they have protected the intrusive sheets, which are therefore 
less deeply cut away than in any of the other districts, and no great central 
core of gabbro has yet been uncovered. 
1 “ Geology of the Henry Mountains,” by Mr. G. K. Gilbert, U. S. Geographical and Geological 
Survey of the Rocky Mountain Region , 1877. 
