488 ME. E. E. L. DIXON AND DE. A. VATTQHAN ON [Nov. ICJII, 



Lithological characters. — In descending order : — 

 S 2 : — 2. Modiola phase. Chiefly 'pisolites/* 1 oolites, forami- 

 niferal limestones, and calcite-mudstones, the latter in- 

 cluding the unfossiliferous white-weathering variety called, 

 from their porcellanous appearance, ' chinastone-limestones' ; 

 fossils abundant (individuals, not species), notably Seminula 

 in beds with a dark, fine-grained matrix ; some beds dolo- 

 mitized. Contemporaneous erosion is evidenced in some of 

 the oolites by irregular fragments, also of oolite, measuring 

 up to 2 feet in length ; in such beds dolomite, if present, 

 is more abundant in the matrix than in the fragments. 

 1. Standard limestones. Oolites, with some pisolites 

 and fine-grained limestones ; in many, few macroscopic 

 fossils, but others are coral-, 2 brachiopod-limestones, etc. 

 Contemporaneous erosion is evidenced in the same way as 

 in the overlying group. Dolomites recur at intervals. 

 S : — Limestones, largely made up of crinoids, corals, brachiopods, 

 and, in some dark impure beds, gasteropods ; dolomitic beds 

 at intervals ; chert-nodules and beekitized fossils, extending, 

 with the gasteropod-beds, possibly into the base of S 2 as in 

 the South- Western District. 



Fauna. — See pp. 547-48. The Modiola phase has not yielded 

 any of the peculiar forms generally present in such phases. 



Thickness. — Of the whole zone, about 1250 feet (Longland Bay). 

 The subdivisions were not separately measured, but S 2 was by 

 far thicker than S r 



Limits. — The upper limit of S was not examined. The two 

 parts of S„, though separable in a broad way, pass insensibly 

 one into the other, the incoming of the pisolites abundant in the 

 upper part being quite gradual. The upper limit of S 2 is marked 

 by a rapid and considerable lithological and faunal change, thus : — 



Junction of S 2 and D r — Generalized from several exposures. 



D t : Pseudobreccias of limestone or dolomite, with a typical D x -fauna from 



the base upwards ; in places a thin basal oolite. 

 Passage-beds: Current-bedded, fine-grained, and in part oolitic limestones 



with Cyathophyllum ; 15 to 20 feet. 

 S 2 : Modiola phase. Limestones of the types previously mentioned; 



Seminula ficoides in crowds, at intervals up to the top. 



1 Much of the ' pisolite' might more properly be termed 'contemporaneous 

 breccia,' for it consists largely of more or less irregular masses, of various sizes, 

 of dense, amorphous-looking limestone, some only of which show a pisolitic 

 structure. These latter consist of a layer of dense calcite, of fairly uniform 

 thickness, around an organic nucleus, generally a Seminula (double-valved) or a 

 coral-fragment, but they rarely, if ever, have a concentric structure of several 

 well-defined layers, though such a structure is frequent in the S 2 -pisolites of 

 the Avon. Under the microscope it is seen that the dense material may contain 

 Girvanella-like tubules, but also that much of it is structureless and of ex- 

 ceedingly fine grain, being devoid, apparently, of any necessary connexion with 

 these tubules. 



2 None of the coral-limestones are unbedded through great thicknesses, and 

 thus differ from coral-reef limestones. 



