

192 EEV. J. F. BLAKE ON THE 



cause the lower part is so full of Serpula gordialis as to almost 

 merit the title of Serpulite. It also contains abundance of Ostrea 

 multiformis. It serves as a guide in strictly local correlation, 

 though it would be absurd to suppose all Serpulites, even if of the 

 same species, to be necessarily continuations of the same deposit. 

 To this portion, which is more free from flints, a thickness of 6 feet 

 may be assigned. 



'No. 8. This constitutes the main mass of the flinty series ; the 

 layers are irregularly spaced and of various thicknesses; some 

 portions of these are false-bedded ; and it is more chalky than the 

 beds above No. 6. JFossils are rather scarce ; but on the surface of 

 the blocks are fucoid markings, and there are numerous specimens 

 of rather large Ammonites — the Am, holoniensis of De Loriol, which 

 is easily distinguished from Am. giganteus by its shape and the re- 

 gularity of its rib-bifurcation. Here also we find associated with 

 Trigonia qihhosa the T. incurva., which, as it does not ascend into 

 the higher beds, indicates a rather lower horizon everywhere. 

 There is also a broad Perna, like P. Flambarti. These fossils are 

 chiefly found in the block at the base, which is more brashy, and 

 has a thickness of 1 foot 8 in. Including this the series is about 

 26 feet thick. 



No. 9 is a shell-bed. It abounds in small oysters and Serpidce ; 

 but Trigonice are not more abundant than other fossils. As it 

 supplies us with the fauna of the flinty series, it is of great 

 importance, though it has been by no means thoroughly searched. 

 The most noticeable are : — Ammonites pseudogigas, which has great 

 nodosities like A. gigas, but they are more numerous ; A. triplex, 

 a very characteristic form ; Pleurotomaria rugata and P. Mozeti, 

 the latter being met with in the so-called Middle Portlandian 

 of Boulogne; Cardium dissimile, which thus appears to mark 

 a lower than the highest horizon at Portland ; Cyprina eJongata, a 

 new species', but found elsewhere ; Lima rustica, Pecten lamellosus^ 

 Trigonia gibbosa and T. incurva and probably other species ; and in 

 places abundance of Pleuromya tellina, whose place in the series at 

 Portland it is important to flx. The thickness is 7 feet 6 inches. 



These nine are the Portland Stone ; and it appears that they form 

 two subdivisions, separated from each other by their stratigraphy, 

 lithology, and fossils, though forming part of the same general 

 series. The upper may be called the Building-stone, and the lower 

 the Plint-beds. The total thickness of the former may be put at a 

 maximum of 12 feet, while the latter is about 68 feet, and is divi- 

 sible into two slightly unconformable masses. 



The rocks which succeed the Portland Stone in a downward di- 

 rection, beyond being called the Portland Sand, have been but little 

 noticed, though they are not inaccessible or uninteresting, and aff'ord 

 valuable means of correlation. 



No. 10 is a stifl" blue marl of a thickness of from 12 to 14 feet, in 

 which I have not succeeded in finding any fossils ; but which from 

 its mineralogical character (so difl'erent from that which one might 

 expect from the title Portland Sands), and from similar occurrences 

 elsewhere, is an important bed. 



