THE AVONIAN OF THE AVON GORGE 
95 
but, by weathering, the surface becomes coated with a thin 
white skin. This peculiar type is found at various levels 
throughout the Seminula-TjonQ of the South-Western Province 
but I have not yet met with it outside that zone. 
Another noteworthy feature of the beds near the north end 
of the quarry is the fact that they are, in several places, stained 
black by petroleum ; one such large patch can be seen in the 
quarry-face, just above the slope on which stand the rifle-butts. 
Plate V. 
Plate IV. 
Syringothyris-ZonQ (C). 
Leaving the Great Quarry at its extreme corner and 
returning to the railroad, the beds immediately below 
are exposed in a low cutting by the side of the line. Here, shales 
predominate, but thick beds of pure dolomite are intercalated 
at frequent intervals and there are one or two beds of oolitic 
limestone. 
The cutting ends in the striking white oolite of the 
‘ Gully Quarry.’ The upper part of this quarry con- 
sists of a thick band of pure white oolitic-hmestone in which 
bedding planes are very inconspicuous, but in which vertical 
joints, traversing the whole thickness of the band, present a 
striking feature. The highest beds of the quarry consist of 
shaley, thin-bedded limestones which form a striking cap to the 
massive band of oolite beneath.'. 
The oolitic band rests upon a series of limestones which are 
rich in carbonate of magnesia ; this series has been termed the 
‘ /ummosu-dolomites ’ on account of the chemical composition of 
its beds and their zonal position. 
If we examine the two slopes of the broad depression known 
as the ‘ Gully,’ we shall find that, at the bottom of the Gully, 
the Oolite lies wholly on the south side, whereas the northern 
slope is a dip slope in the ‘ dolomites.’ Near the top 
of the Gully, however, there is a fine exposure of the white 
Oolite on the northern side, and the screes from this crag ex- 
tend for a considerable distance along the floor of the Gully. 
The massive white Oolite forms so striking a feature in the 
