DURING THE TERTIARY PERIOD IN THE BRITISH ISLES. 143 



The structure and history of the gabbro bosses of the Inner Hebrides find a close 

 parallel in those of the Henry Mountains of Southern Utah, so well described by Mr 

 G. K. Gilbert of the United States Geological Survey. In that fine group of mountains, 

 rising to an extreme height of 5000 feet above the surrounding plateau, and 11,000 feet 

 above the level of the sea, 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 communicated 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 has left records of every 

 stage in its sculpture. In one place, are seen only arching strata, the process of erosion 

 not having yet cut down through the dome of stratified rocks into the trachyte that was 

 the cause of their uprise. In 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 concealed.* 



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 Eum it has been so completely removed, that only a few 

 fragments remain at some distance from the cone 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 less deeply cut away there than in any of the other 

 districts, and no great central core of gabbro has yet been uncovered. 



IV. THE ACID KOCKS. 



We now come to the consideration of the last and in some respects the most singular 

 phase of volcanic action during Tertiary time in Britain. Hitherto all the igneous rocks 

 that have been under consideration in this memoir, whether injected below or poured 

 out at the surface, have been of basic, some of them indeed, like the peridotites, of ultra- 

 basic character. But we now encounter a great series, every member of which is more 

 or less decidedly acid, and in which the excess of silica is very commonly visible to the 



* " Geology of the Henry Mountains," by G. K. Gilbert, U. S. Geographical and Geological Survey of the Rocky 

 Mountain Region, 1877. 



