1848. | WESTON ON THE GEOLOGY OF RIDGWAY. 259 
clearly belong to the same stratum, although the latter is placed 
from circumstances in a very peculiar position. Before speaking, 
however, of this position, we must deal with the formations inter- 
vening between it and the greensand, taking them in their descending 
order. 
We may infer a greater extension of the Hastings sands and clays 
in former times than what has hitherto been observed in the Wey- 
mouth district; and the Purbeck beds at Ridgway and Portland 
doubtless existed continuously anterior to the protrusion of the great 
boss of forest marble. But from our present knowledge of all the 
freshwater beds deposited between the oolitic and cretaceous systems, 
there is no necessity for supposing that the Hastings sands and Pur- 
beck beds ever dipped northward under the Ridgway chalk. The 
Wealden formation in the aggregate is essentially a local deposit. 
In considering, too, the whole upper system of the oolitic group, 
we cannot but be struck with ‘‘the sudden and total disappearance 
of the Portland formation at Portisham,” which, according to the 
views of the authors of the paper referred to, ‘ seems not to result 
from its accidental intersection at that place by the great Ridgway 
fault, but rather from a tendency, which is common to this with most 
other great formations, to terminate abruptly where they are accu- 
mulated to their fullest thickness.” The reappearance and equally 
abrupt termination ‘‘of the Portland formation” are also to be seen 
‘in the Vale of Tisbury and near Brill and Thame im Oxfordshire, 
and near Aylesbury and Whitchurch, Bucks, and near Swindon, 
Wilts.” 
These valuable observations, and many others on the same point, 
lead us to feel that the Portland formation, and indeed the whole 
upper oolitic system, may terminate abruptly on the north as well as 
on the west of our locality, and therefore that this system, conjointly 
with the Wealden, need not dip under the chalk of Ridgway. If 
this reasoning be deemed satisfactory, we shall have relieved our 
subject from great difficulty. 
The existence or non-existence of the Oxford oolite between the 
greensand and the Oxford clay beneath the chalk escarpment is of 
no great consequence. I think, however, as it is developed north 
and south of the geological saddle of forest marble, that most pro- 
bably it does pass northward under Ridgway Hill, immediately be- 
neath the greensand. 
Having considered all the strata mterposed between the Oxford 
clay and the Ridgway chalk, we are now prepared to examine the 
actual circumstances of the Oxford clay im the Ridgway cutting. 
This clay is clearly not the Oxford clay stratum in its natural posi- 
tion, but a part of it forced out and raised above its own level by local 
pressure*. It thus stands hke a trap-dyke intruded between the 
almost perpendicular walls of chalk and Hastings sand. 
* The finished line of the railway in this cutting is about 247 feet above the 
level of high flood-tides at Weymouth, and the shaft sunk from the surface-line to 
the level is about 50 feet. The Oxford clay at Ridgway must therefore be about 
300 feet higher than that near Weymouth. 
