Geologists' Association. 93 



on the coast. The Dogger proper is poorly developed here, as are 

 the lower shales and sandstones ; but the second fossiliferous zone of 

 the Lower Oolites is well represented by an Oolitic Limestone of 

 considerable importance, containing several species characteristic of 

 the lower part of the Inferior Oolite. Above this, and separated by 

 a few feet of sands, are some arenaceous limestones containing Lima, 

 hellula, PJioladomya Theraultii, Ehynchonella spinosa, a large discoidal 

 Ammonite, etc., perhaps on a level with the Trigonia-grits of 

 Cheltenham. This fauna gradually dies out through excess of sandy 

 matter, some shales intervene, and then another group of sands with 

 siliceous limestones occur. These have a tolerably abundant fauna 

 of a peculiar character. Avicula, Gervillia, and Myacites are es- 

 pecially abundant in these beds, which may possibly be the equivalent 

 in time of the Cornbrash of the coast, though more estuarine in 

 character. 



II. — January 2nd, 1874. Henry Woodward, Esq., F.E.S., etc., 

 President, in the Chair. " On the Nature and Formation of Flint 

 and allied bodies." By M. Hawkins Johnson, Esq., F.G.S. 



The object of the paper was to show the nature of several members 

 of a large group of bodies occurring in sedimentary deposits of dif- 

 ferent ages, and which are generally known as nodules, and described 

 as concretionary. Those specially alluded to were the Septaria from 

 the London and Kimmeridge Clays, the Flints from the Chalk, Iron 

 Pyrites from the Chalk, the Phosphatic nodules of the Gault, the 

 Clay Ironstone nodules of the Carboniferous series, and the Ironstone 

 from the Woolwich Beds. By the gentle action of solvents, the 

 structure of these bodies is revealed so as to be easily examined by 

 the microscope. They are then found all to agree in possessing 

 a silicified organic structure, which may be described as a network 

 of fibres, or a mass permeated in every direction by anastomosing 

 canals. This structure was subsequently filled in with other 

 material, such as carbonate of lime, silica, bisulphide of iron, 

 phosphate of lime, carbonate of iron, etc., the particular substance 

 thus filled in depending upon the relative abundance of the sub- 

 stances dissolved in the interstitial water of the surrounding matrix. 

 The singular group of concentric siliceous circular bands seen upon 

 many fossils, and known as Orbicular Silica, or Beekite Markings, 

 were also referred to. The fossils on which they occur were im- 

 bedded in a matrix more porous than themselves, and of irregular 

 constitution, so that the evaporation, to which the consolidation of 

 the dissolved silica in their pores was mainly due, occurred at a 

 number of points on the surface of the fossil, at which points 

 a deposit of silica took place, forming the central tubercles. The 

 cessation of evaporation was followed by a fresh saturation with the 

 solution, to be again evaporated. But as the evaporating points 

 were now plugged up by the previous deposits, the silica last con- 

 solidated was deposited around their margin and upon them in- 

 ternally, appearing outwardly as a ring round the tubercle. Alter- 

 nations of these conditions account for the numerous bands seen in 

 some of the groups. 



