Dr. FiTTON on the Strata below the Chalk, 



253 



Ft. In. Ft. III. 



7. (11. of Cliicksgrove.) Uniform, soft, very white limestone, here called' 

 "chalk"; but abounding at the upper part in Portland fossils: Pecten la- 



mellosus, Cardium dissimile, Trigonia gibbosa, Ostrea expansa. This bed is 

 not fit for building, and is burnt into lime : it seems, nevertheless, to represent 

 the " white-bed " of Portland, though without oolitic structure 



8. (12., &c., of Chicksgrove.) From 7. to the bottom of the quarry, a depth of 

 about 25 feet, in 8 or 9 beds, is called " Freestone " : the detail as follows : — 



a. Whitish stone, called " chalk ", of uneven, subconchoidal fracture ; con- 1 „ 



taining great numbers of Pecten lamellosus and Cytherea / 



6. Cream-coloured, somewhat sandy stone, also with Pecten lamellosus .... 2 



c. Similar stone ; Trigonia gibbosa, Cytherea, Pecten lamellosus. Sometimes 1 „ 

 this bed includes 5 to 8 inches of sand 2 ft. to J 



d. Concretional stone and sand. 



i. Stone tol 8l 



ii. Sand, separating and surrounding the concretions of i. 4 to 2 Q \ 



about 

 e. Stone and sand : — 



i. Concretional stone 1 



ii. Sand around the concretions 2 



4. 



to 3 

 to 



1} 



/. Stone 4 



g. Ditto 3 



h. Ditto 2 



25 



These lower beds (which afford the best stone) contain Ammonites 

 giganteus of great size : they are more like the form of the Portland stone 

 which occurs in the Isle of Purbeck than that of Portland Isle itself. 

 Bottom of the Quarry. 

 At a new quarry near Chicksgrove Mill, the dip was east 25° south, at an angle of 2° or 3°. 



(133.) Junction of the Purbeck and Portland formations . — It is obvious that 

 the preceding strata, down to 7 , represent the lower part of the Purbeck se- 

 ries, which in the Island of Portland includes the petrified trees and Cycadeae ; 

 and the section here is valuable, as it exhibits some variation in the form of 

 those lacustrine deposits. One of the principal points of difference between 

 the sections at the two places is, that the freshwater strata here rest, imme- 

 diately, upon a bed containing marine fossils, without the intervention of any 

 clay or dirt. Another difference is, that no flinty masses are found here, at 

 the top of the Portland formation, like those immediately above the white 

 bed in the Isle of Portland; but, on the contrary, a range of irregular masses 

 of black flint occurs distinctly within the freshwater series, and is separated 

 from the Portland stone (No. 7. of the preceding list) by a bed of freshwater 

 limestone (No. 6.), including casts of Cypris. As the flint, in this latter 

 case, does not contain fossils, its presence immediately above the junction, 

 while it is wanting in the usual place below, might at first sight mislead : but 

 the occurrence of flinty masses is by no means unfrequent in other parts of 

 the Purbeck group ; and in the quarries north of the Nadder, opposite to 



