274 J. F. BLAKE AND W. H. HTTDLESTON ON 



Pleuromya donacina (Ag.). 

 Cardium delibatum (Be Lor.). 

 Inoceramus, sp. 

 Pinna pesolina (Cont.). 

 Pecteii midas (B'Orb.). 

 Rhynchonella corallina (Leym.). 



inconstans (Sow.). 



Terebratula subsella (Leym.). 

 Waldheimia lampas (Sow.). 



dorsetensis (Bav.). 



Echinobrissus scutatus (Lam.). 

 Glyphaea ferruginea (Bl. cf H.). 

 Serpula Poyeri (Be Lor.). 



gordialis (Gold/.). 



Wood. 



Ichthyosaurus (tooth). 

 Ammonites decipiens (Sow.). 



hector (B'Orb.). 



Belemnites nitidus (BoVf.) 

 Ohemnitzia pseudolimbata (Bl. <$[H.). 



delia (B'Orb.). 



■ ferruginea (Bl. $• H.). 



Pleurotomaria reticulata (Sow.), var. 



Alaria, sp. 



Pterocera, sp. 



Natica eudora (D' Orb.). 



Modiola subagquiplicata (Gold/.). 



Area sublata (I? Orb.). 



Trigonia monilifera (Ag.), var. 



Pleuromya Voltzii (Ag.). 



tellina (Ag.). 



Some of the fossils are so badly preserved that we are unable to 

 identify them with certainty; and among these arc species of Pterocera, 

 which are of great interest as possibly indicating a deposit similar 

 to those known as " Pterocerian " on the Continent, but not else- 

 where found in England. The other great feature of the fauna is 

 the presence of Brachiopoda, which we have nowhere else met with 

 in the Corallian beds of this district; and from the association of 

 Rliynchonella corallina with Ii. inconstans we may infer that we are 

 really dealing with beds at least on the horizon of the passage-beds to 

 the Kimmeridge Clay; and the other fossils lead to the same conclusion. 

 .Combining this with the stratigraphical evidence, we may arrive at 

 a pretty correct estimate of the age of this Abbotsbury Ironstone. 



The remarkable structure of this rock requires more than a 

 passing notice. It has undergone much chemical change since its 

 deposition ; and this has caused the decortication and destruction of 

 the more tender fossils. Except where seen unweathered, as in some 

 of the lower beds, it has not usually much coherence; so that when 

 dried and pressed the particles readily separate ; and we find it to be 

 made up of three principal constituents : — 



1. Quartz grit, rounded and subangular, and variable in size. 

 , 2. Coffee-coloured granules with a smooth surface, for the most 

 part circular and flattened, but occasionally oval, about the size of a 

 large pin's head, with fragments of the exterior scattered about like 

 broken egg-shells. 



3. A buff-coloured investment, much poorer in iron than the 

 granules, which adheres to the quartz grains, and sometimes forms a 

 cement to the whole mass ; it contains a considerable amount of 

 ferrous carbonate. 



The presence of so much silica in the form of quartz is said to 

 detract from the commercial value of the ore ; but the small amount 

 of phosphoric acid found in the granules may possibly compensate 

 for this. 



The granules, when broken, are seen to consist of a brown exterior 

 shell, which is largely but not wholly composed of ferric hydrate, and 

 of an interior filled with a buff- coloured powder much poorer in iron. 

 There has been a centrifugal movement in the iron here, such as is 

 known to occur in many peroxidized iron-ores ; but this is perhaps a 

 comparatively recent action. The granule may perhaps be called " an 



