14 S. &. EMMONS 
to determine accurately the structure of the middle of this 
basin, where the surface is marshy or shows only outcrops of 
igneous rocks, and upon this subject much has been written. 
Near Johannesburg the dip of the beds is generally 45° to 80° 
at the outcrop, though in a few instances not over 25°. This 
inclination decreases with depth, quite rapidly though with no 
uniformity, and at vertical depths of 500 to 1000 feet, it has 
usually become 30° or less; the borings show a probable angle 
as low as 12° at greater depths. Should this decrease continue 
_with sufficient rapidity, the whole basin, even at its deepest part, 
might be within the limits of profitable mining. Gibson, who is 
inclined to extreme views in regard to structure indications, says 
that the surface of the foot and hanging walls of the “reefs” or 
“bankets”’ are smooth and polished; the pebbles flattened, and 
sometimes completely shattered; the cementing material 
decidedly schist-like and squeezed in and out around the pebbles. 
He remarks further that the conglomerate beds are generally 
found to decrease in number as greater depth is reached. All 
these facts he regards as evidence of strong compressive move- 
ment combind with overthrust faulting. The apparent rapid 
decrease of angle of dip in depth he seems to think due to 
repetition by reversed faulting and he hence opposes the theory 
of a simple basin structure. The igneous rocks he regards as 
much later than the conglomerates, and probably later than the 
movement of compression, since the diorite intrusions show no 
effects of it. 
Molengraaf, on the other hand, does not consider that there 
is any considerable folding in these beds, and thinks Gibson’s 
proof of overthrust faulting weak. He admits that both strike 
and dip faults are common, and that there are evidences of 
movements within the beds; he considers that these result from 
thrust faults with movement from north to south instead of 
from south to north as Gibson maintains. The rapid shallowing 
of the dip he thinks easily explained if the steep upturning of 
the beds around the rim of the basin is considered due to 
dragging of the strata over each other. 
