THE QUIZZYHOTA LACCOLITE 81 
in the sequel that a considerable proportion of the Karroo dolerite 
is composed of the melted-up sediments, and that the accession of 
fresh molten material from the deeper portions of the crust was 
comparatively small in proportion to the amount exhibited on the 
surface. 
THE GREAT KARROO LACCOLITE 
All the dolerites of the Karroo—the sills and dykes of the main 
portion and the laccolites on the east—belong to one and the same 
system; they together form one immense laccolite of the type 
known as the ‘‘ Cedar tree laccolite.” The type was first described 
by Holmes on the La Plata Mountains of Coloradot and has since 
been noted in the gabbro of the Cuillin Hills of Skye,? but nothing 
like the stupendous nature of the Karroo laccolite occurs in other 
parts of the world. The Karroo laccolite is some 700 miles long 
by at least 200 miles broad; the great Transvaal laccolite, large 
as it is, is only 250 miles long. The dolerite sills form generally 
in the upper portions of the great laccolite, whereas the lumps and 
expansions forming the subsidiary laccolites, of which the Quizzy- 
hota is one, occur either along the central axis or in the lower por- 
tion. In Komgha one is, as it were, in the cellars of the giant 
structure; the upper stories can be seen to the north in the long 
table-topped hills of Cathcart. Thence to the north the sills follow 
each other in countless steps up to the main watershed of the 
country, which, for a great distance in the west, is a steep escarp- 
ment facing south, and generally some 6,000 feet above sea-level, 
though exceptional heights occur on the north of the Graaff Reinet 
Division in Compass Berg, 8,500 feet, and Middle Mount, 6,263 
feet. East of Compass Berg the main watershed and the central 
axis of the great laccolite continue in the same direction, but the 
escarpment curves to the southwest. All these sills dip slightly 
inward to the north; they are connected by innumerable dykes and 
often branch and undulate in many remarkable ways. North 
of the main watershed all the drainage flows into the Orange 
River, which is held up by the bar across its lower course at the 
t Ninth Ann. Rept. U.S. Survey Territories, Washington, 1877, XLV, Fig. 2. 
2A. Harker, Nat. Hist. Igneous Rocks, 67; “Tertiary Igneous Rocks of Skye,” 
Mem. Geol. Survey of Glasgow, Fig. 16, p. 80. 
