LIMESTONES. 
31 
but the introduction of foreign marbles lias occasioned its use to 
be almost discontinued. Very similar to this Purbeck marble 
are certain fresh-water limestones abounding with paludince, 
occurring in thin local bands in the Weald clay, and known as. 
Petivorth, Bethersden, and Sussex marble . 
The beds immediately below the Purbeck series are termed 
the Portland beds, since they are typically developed in the 
peninsula near Weymouth, known as the Isle of Portland. These 
strata yield some of the finest freestone in the country. The 
highest bed, under the Purbeck “ skull cap,” yields the stone 
called Roach ; this is a tough strong stone usually presenting 
a cavernous structure, due to the removal of fossils which it 
once contained, some of the commonest of these organic relics 
being the quarrymen’s “ screw-stones,” or casts of Gerithium 
PorUandicum. Under the roach is the Whit bed, 8 or 10 feet 
in thickness, furnishing some of the best stone in the series; 
and at a yet lower horizon is the Base bed, which, though less, 
durable, is also valued as a freestone, being indeed sometimes 
known as the “ best bed.” A rough inferior stone called bastard 
roach, and certain cherty limestones likewise occur in the Port- 
land quarries. It should be mentioned that Portland stone is 
quarried, not only in Portland, but in the Isle of Purbeck, while 
varieties are also obtained in the Vale of Wardour and at 
Swindon. A large Ammonite (A. gigantens) characteristic of the 
Portland beds will be found at the base of the cast of the Dying 
Gladiator, on the opposite side of the Hall (No. 106). 
Previously to 1623 Portland stone does not appear to have 
attracted much attention. From 1660 it has gradually grown 
into use. Inigo Jones restored a portion of Old St. Paul’s, 
“ casing great part of the outside, and adding a grand Corinthian 
portico to the west front, all of Portland stone.” St. Paul’s. 
Cathedral and many of the city churches and other large 
buildings erected in the reign of Queen Anne, were constructed 
with stone very superior to that now generally employed as far 
as regards durability. The quarries from which Sir Christopher 
Wren obtained the Portland stone which he employed have been 
long deserted, the reason assigned being that the stone is a little 
harder, and thereby more expensive to work. 
The Portland stone belongs to the Upper series of the Oolitic 
strata, or those which lie between the Cretaceous formation 
above and the Lias below. The name a oolite ” (Greek, oon, an 
egg, lithos, a stone), is derived from the limestones of this group 
being, for the most part, made up of egg or roe -shaped particles, 
which are spheroidal concretions of carbonate of lime, each grain 
usually presenting a concentric structure, and enclosing a particle- 
of sand, or some other substance, often organic, which has served 
as a nucleus. The Portland limestone is in parts distinctly 
oolitic, but it should be remembered that all the limestones, 
belonging to the oolitic series do not present this peculiar 
texture, nor, on the contrarjq is such oolitic structure confined 
