84 ; E. H. L. SCHWARZ 
remnants left in the Worcester and Robertson Divisions—are not 
intruded by dolerite. 
The great system of sills and dykes runs out to sea with the 
Karroo rocks near East London and comes in again with them near 
St. Johns. From the last place the margin runs north to Pieter- 
maritzburg and Ladysmith, thence across the north of the Orange 
Free State down the Vaal River to Hope Town, Prieska and Brand 
Vley, thence through Calvinia to near Karroo Poort in the Ceres 
Division and from there to Beaufort West. ; 
The Stormberg lavas have nothing to do with the Karroo 
dolerites which the volcanic chimneys pierce. Dolerite dykes 
indistinguishable in composition from the Karroo dolerites traverse 
in turn the lavas of the Drakensburg, but these latter are rather 
to be regarded as dykes connected with the intrusion of the lava. 
The dolerite as a whole is an augite-olivine-plagioclase rock; the 
varieties with a little quartz in granophyric intergrowth or with 
mica are quite insignificant. One peculiar glassy dyke with nephe- 
line I obtained in Beaufort West,’ but otherwise the normal dolerite 
is of an intensely conservative nature. A diorite often traverses 
the rocks in the east at Cradock and in Komgha and Kentani.? 
These I shall have to refer to later; they are usually taken to be 
later injections from a primitive magma which has rid itself of the 
more basic substances, but I shall endeavor to prove that they are 
the channels down which the more acid materials absorbed by the 
dolerite from the country rocks are passed out. 
THE PROBLEM OF THE DOLERITE INTRUSIONS 
Having now established the relationship of the Quizzyhota 
laccolite to the dolerites of the Karroo and that of the sedimentary 
rocks to those forming the main portion of the Karroo, we have 
to inquire how a great mass of igneous rock can have come to lie 
within the sedimentary rocks without disturbing their stratification 
and without notably altering them by thermal metamorphism. 
*E. H. L. Schwarz, ‘‘Geological Survey of the Beaufort West District,’ Ann. 
Rept. Geol. Comm., 1896, Cape Town (1897), 18. 
2A. W. Rogers and E. H. L. Schwarz, ‘“‘General Survey of the Rocks of the 
Southern Transkei,’ Ann. Rept. Geol. Comm., r901, Cape Town, 1902; ‘“‘The Transkei 
Gap,” Trans. Phil. Soc. S. Africa, XIV, Cape Town, 1903. 
