Dr. FiTTON on the Strata below the Chalk. 251 



Some of the beds of uniform whitish limestone, to which the workmen give the name of "Lias", 

 very much resemble the stone of Monte Bolca; and where most compact, are so like the stone 

 used in lithography, that their application to that purpose seems to be well deserving of trial. 

 The exposed edges of the thicker folia are eroded deeply into spongiform cavities. In some of 

 the pits, the limestone, alternating with clay, has the appearance of united concretions or lumps ; 

 but within the stone is uniform, the aspect that of freshwater limestone, and the fracture 

 splintery or very flat conchoidal, resembling a rock found in a corresponding place at Gar- 

 sington near Oxford, which is there called " Malm ". The masses split into irregular flakes at 

 right angles to the surfaces, not more than a twentieth to an eighth of an inch thick. Both the 

 clays and limestones in this part of the Purbeck series have sometimes a bluish colour ; and 

 both inclose large specimens of Ostrea distorta. 



On the north side of the valley, as the Purbeck strata rise more rapidly, the sections are not 

 so much expanded; but about TefFont Evias slaty limestone, alternating with clay and fibrous 

 carbonate of lime, is seen not far from the gault, dipping about north 15" west, at an inclination 

 between 2° and 4°. A thick coating of these strata invests the heights thence to Ladydown, 

 (see the section, PI. X. a., fig. 13, ab.); but about midway to that place, it is cut through by a 

 natural ravine, which runs from Chilmark to the Nadder at Penthurst Bridge, and not impro- 

 bably may have originated in a dislocation. The Portland beds which form the lower part of 

 the ravine have been extensively quarried on both sides. 



At Ladydown, near the highest ground occupied by the Purbeck formation, quarries have long 

 been worked to great extent, chiefly for the sake of the fissile cycladiferous stone, like that of 

 the Isle of Purbeck, which here bears the name of "Tilestone ". Among these beds is a group 

 which abounds in the remains of fishes. 



On the south of the Nadder, near Benston, a prominence, like a step at a lower level, extends 

 beyond the sands and clay which form the upper part of the hills, and is continued to Wockley 

 and the west of Anstey Water, capping the height occupied by Totterdale Farm, where the Pur- 

 beck beds run out. In the lower part of this projection, a large quarry was for many years worked 

 at Chicksgrove Mill, and another more recently at Wockley ; the bottom of the former is nearly 

 level with the river ; the top about 50 feet above it, and about 30 feet lower than the top of 

 the prominence above mentioned. The total thickness of the Purbeck strata indicated by these 

 data does not exceed 60 feet. 



A few beds occur in the upper part of the section at Chicksgrove, which do not remain at Wock- 

 ley ; but the rest are so nearly the same at the two places as not to require a separate detail. 



List of the Beds at Chicksgrove Mill, and Wockley Quarries. 



Ft In. Ft. In. 

 Chicksgrove, — 



Grass and soil about 9 



1. Loam; including fragments of stone, like the "lias" of the pits above men-"l 

 tioned about j 



2. Bluish clay, passing into 3 3 in. to 6 in. 5 



3. Yellowish brown, tough clay, very uneven at the bottom; filling up the ir-1 « (. 

 regularities of 4 3 in. to 9 in. J 



4. A bed of coarse, somewhat gritty limestone, very irregular in thickness, but ■] 

 nearly continuous ; in many places having the aspect of magnesian limestone, V2 



and containing nests of carbonate of lime in ihombic crystals 1 ft. to 3 ft. J 5 



