Mr. Webster on the Purbeck and Portland Beds. 41 



found nearly entire. I have also seen fins of the balistes, and teeth of the same 

 species offish as I have mentioned to be found at Hastings. (See p. 35.) 



As the thickness of all the formations below the chalk diminishes in a very 

 remarkable manner in proceeding from the east end of the Isle of Purbeck 

 westwards, so the beds that we have been considering partake of this change; 

 for although the quarrymen can point out, in the quarries to the west (as, for 

 instance, in one at Warbarrow), beds which they consider to correspond to 

 the veins which they work at Swanwich, yet the greatest part of the beds to 

 which they give names at the latter place do not exist westwards. At the 

 quarries of Tillywhim, Warbarrow, and Mewp Bay, the junction of the pro- 

 per Purbeck beds containing fragments of shells, with the beds analogous to 

 them in the upper part of the series in the Isle of Portland, to be afterwards 

 described, may be advantageously examined. 



I shall next proceed to point out what I have observed chiefly remarkable 

 in the Isle of Portland. 



The principal quarries in this place were formerly on the north-east side; 

 but these are now almost abandoned, and they procure the stone more conve- 

 niently on the highest part of the island at the north end. These quarries are 

 not worked by galleries, but are all open to the day. The section of one of 

 the principal quarries on the north end is as follows*. (See Plate VI. fig. 3.) 



Immediately under the soil, which seldom exceeds a foot in depth, is a 

 series of thin beds, all together about 3 feet thick, called slate by the quarry- 

 men, which split readily into layers from an inch to half an inch in thickness. 

 They consist of limestone of a dull yellowish colour, extremely compact, and 

 entirely without shells (at least I have not seen any in it). In its aspect it con- 

 siderably resembles those compact varieties of Purbeck stone in which the 

 remains of shells are not visible. These are what I consider to be analogous to 

 the beds in the lower part of the Purbeck limestone, seen at Warbarrow and 

 Mewp Bays above mentioned. 



Below this is another mass of calcareous stone, considerably softer, and of 

 a lighter colour than the preceding : it is divided into two by a slaty bed, the 

 upper being called aish, and the lower the soft burr. The latter stands upon 

 abed, about one foot thick, consisting of a dark-brown substance, and contain- 

 ing much earthy lignite ; this bed is very remarkable, and extends all through 

 the north end of the Isle of Portland; I even found some traces of it in the 

 coves at the west end of Purbeck : it is called by the quarrymen the Dirt bed. 

 In it are found considerable numbers of fossil trunks of trees of the dicotyledo- 



* Specimens of all these beds I have placed in the collection of the Society. 

 VOL. II. SECOND SERIES. G 



