67 
feet. It may be studied near Sandsfoot Castle, Eingstead Bay, and at 
Portland Island. 
In many places this clay contains so much bituminous matter as to be 
inflammable, and is called Kimmeridge coal. When distilled the bituminous 
schist yields one hundred and sixty three gallons of inflammable oil from 
every fifteen tons. The Kimmeridge clay is very fossiliferous, and may be 
known wherever it is seen, by the presence of the large Ostrea deltoidea. 
It forms the base of Portland Isle, and may often be examined at its 
northern extremity. 
The upper portion of the Island consists of beds of Portland sand and 
stone, capped by a freshwater formation called the Purbeck. These are by 
far the most interesting of all, and by means of the quarries may always 
be easily examined. 
The Portland sand, eighty feet in depth, has a bluish grey colour, from 
small grains resembling those of the Greensand. 
Above the sand are beds of impure cherty limestone, seventy-five feet 
in thickness. About forty feet from the bottom is an extremely hard 
siliceous bed, so crowded with casts of shells, especially Cerithhtm 
Portlandicum, as to be quite unfit for architectural purposes, but it is 
admirably adapted for quay-walls, breakwaters, &c. 
Above this marine series of deposits commence twenty-five feet of fresh- 
water beds. They begin with two most singular remains of ancient land, 
separated from each other by rock called "skull-cap," and themselves 
named the "dirt-beds." It is, doubtless, the actual soil most wonderfully 
preserved, and still contains the bases and roots of the old pines and 
Cycadese. The latter resemble gigantic pine-apples, plainly shewing 
the scars of the leaf-stalks. 
The tree-trunks lie prostrate, but the stumps still stand erect in their 
original position with the roots penetrating the underlying stone. Strangely 
enough they are almost entirely composed of Silica. The concentric rings 
of growth, knots, and other markings, are most distinctly preserved. On 
the opposite side of Weymouth Bay the dirt -bed may again be seen, but 
dipping at a much greater angle. 
The true building stone is soft and white, and when struck with a 
hammer, rings with a musical note. It is worth in the quarry twenty 
shillings per ton. 
The other beds of the Purbeck formation may be better studied at 
Lulworth Cone, where the incursions of the sea have made splendid 
sections. 
The paper was illustrated by a numerous suite of fossils, obtained from 
the several spots described, and a number of diagrams of parts possessing 
the greatest scientific interest. 
