252 PROCEEDINGS OF THE GEOLOGICAL society. [Feb. 23, 
Purbeck beds, which, though long known, have by the excavations 
of the railway carried on south of (sm) in the section Fig. 1 been for 
the first time so completely laid bare for geological examination. 
The Portland sand I traced on the other side of the Upway valley, 
and I learned that it had been previously denuded on the north side 
of the high road at a 6 Fig. 1. by the excavations made for building the 
north foundation of the viaduct. Considerig the inclination of the 
Portland stone strata and the breadth of the valley, the Portland sand 
may be considered as well-developed in this spot. 
The superjacent Portland stone is clearly defined in its limits by 
the arenaceous deposit below and the lowest of the Purbeck dirt-beds 
above. I found an ammonite in the lower part of this marine for- 
mation, and one of the superintendents of the works poimted out to 
me the place where a fossil fish, and what he described as “like a 
dog’s jaw,’ were found. This place was in the upper part of the 
Portland stone, and not very far from its junction with the Purbeck 
beds. 
The Portland stone is much less developed in this locality than m 
the Isle of Portland. The Purbeck beds at Ridgway possess, how- 
ever, much greater thickness than the same formation m Portland, 
where even on the northern part of the island they do not attam a 
thickness of more than twenty feet. I do not say “developed,” be- 
cause we have in Portland no Hastings sand formation to mark the 
upper limits of the Purbeck beds, and therefore the absent superior 
strata may have formerly existed in the island and been subsequently 
removed by diluvial action. 
The Purbeck beds now laid bare exhibit many dirt-beds ; these how- 
- ever, do not possess the thickness of the dirt-bed m Portland which 
contains the silieified coniferous trees, but more resemble the thinner 
dirt-bed interposed between the “ Skull-cap”’ and ‘“ Top-cap,” and 
which contains only the compressed Cycadeoidee. 
The upper parts of this formation (which are wanting in Portland) 
contain the remarkable layer of fibrous carbonate of lime as well as 
what is denominated ‘‘ Purbeck marble.’ Almost all the quarries 
on the south side of Ridgway Hill are im the Purbeck formation. 
The Purbeck beds are characterized generally by their schistose 
structure and by the alternate occurrence of argillaceous and cal- 
careous deposits. This alternation, carried on still further im detail, 
affects the composition of the Purbeck stone and imparts to it its 
peculiar streaky or ribbon appearance. I might add also that there 
is a remarkable persistency in these characters throughout the for- 
mation, at least from Upton, near Osmington, to Portisham*. 
* Connected with Osmington I would call attention to the sections Figs. 9 and 3 
cf Plate 2 in the paper on the geology of Weymouth. Fig. 9 very correctly shows 
the arched strata of Osmington Mills running east and west, and Fig. 3 one-half of 
the arch running north and south. I found, however, on examining the shore at 
low-water, that the Oxford oolite washed by the waves decidedly dipped to the south, 
completing consequently the south side of the arch. Osmington Mills thus stand 
on the intersection of two arcs, cutting each other at nearly right angles, and seem 
therefore to be on the very focus of the upheaving powers formerly operating on 
this part of the coast. I ought to add, that both on the east and west of this spot 
the strata seen on the sea-shore resume their dip to the north. . 
