156 W. D. Lang—The Selbornian of Charmouth. 
In the absence of Cowstones it is impossible to draw the lithological 
limit between the zones of Hoplites interruptus and of Schloenbachia 
rostrata that is recognized on Black Ven. In the absence of fossils 
the true limit cannot be determined. If the point at which typical 
Foxmould sand begins is taken as the lower limit of the rostrata zone, 
it is obvious (see Fig. 2) that the enterruptus zone is thinner on Stone- 
barrow than on Black Ven. Again, on Golden Cap, Jukes-Browne 
describes some 100 feet of sand (and no Cowstones are mentioned) 
resting directly on the Gault fossil bed, which is only 6 feet thick. 
That is, if lithological characters are taken into consideration in the 
absence of fossil evidence, the interruptus zone above Bed 3 has thinned 
from 25 feet on Black Ven to 11 feet on Stonebarrow and 6 feet on 
Golden Cap. 
If fossils occurred it might be found that the zone had not thinned 
from west to east, but only become more sandy. But the evidence of 
the hard bands (Beds 4, 6, and 8) on Black Ven is against this 
supposition. For it looks as if the 4 inches of loam, indurated in 
places, containing Serpula concava, Sowerby, on Stonebarrow, was the 
equivalent of these hard bands on Black Ven. In that case Bed 3 on 
Stonebarrow would be 7 feet thick, against 10 feet on Black Ven; 
thus showing a thinning of the upper part of the zone eastwards. 
The lower part of the zone remains to be considered. On Black Ven, 
beneath Bed 38, there are about 14 feet of loamy sand before the 
junction with the Lias is seen. On Stonebarrow the junction has 
not been found, and only a foot or two of loamy glauconitic sand is 
shown beneath Bed 3. On Golden Cap about 24 feet, chiefly of 
sands, are exposed beneath Bed 3, according to the interpretation 
here given to Jukes-Browne’s section. It is probable, then, that the 
interruptus zone helow Bed 3 thins from east to west, in the contrary 
direction to the upper part of the zone. Assuming that this thinning 
is regular, and taking two miles as the distance between the Golden 
Cap and Stonebarrow sections, and one and a third miles as that 
between the Stonebarrow and Black Ven sections, the thickness on 
Stonebarrow below Bed 8 would be 18 feet. ‘This would make the 
base of the Cretaceous on Stonebarrow 320 feet above sea-level. 
VI. Summary and Conclusions. 
The chief results arrived at in this paper may be summarized as 
follows :— 
1. The Hoplites interruptus zone is exposed in two small sections on 
Stonebarrow cliff, but the junction with the Lias has not been found. 
2. A fossil bed containing the interruptus zone fauna occurs here, 
westwards on Black Ven and eastwards on Golden Cap, thus giving 
a stratigraphical datum-lne. 
3. That part of the zone above this fossil bed either becomes more 
sandy when traced from west to east, or, more probably, begins to 
thin out. 
4, That part of the zone below this fossil bed thickens from west 
to east. 
5. No Cowstones appear on Stonebarrow, nor have they been 
recorded from Golden Cap. 
