670 REPORT— 1886. 



8. On an Accurate and Hapid Method of estimating the Silica in Igneous 

 Bodes. By J. H. Plater. — See Reports, p. 471. 



9. On a new Form of Clinometer. By J. Hopkinson, F.L.S., F.G.S. — See 



Section A, p. 557. 



10. On Concretions. By H. B. Stocks. 



A concretion may be defined as a more or less spherical mass of rock-matter 

 collected around some organic substance, or upon itself, to form a nodule. 



Concretions occur in many rocks, and vary very much in their composition. 



The follovs'ing are examples : — 



Hydraulic Limestones occur as concretions in the Lias and other formations ; 

 they are of a grey colour and very hard. They are used for the preparation of 

 hydraidic cement. 



Baum Pots are nodules occurring in the shale of the coal-measures'; they are 

 very much like hydraulic limestone in appearance, being hard and of a grey 

 colour. 



Coal Balls are thus named because they occur in the coal ; they are of a brownish 

 colour and nearly spherical. 



Acrespire is a form of concretion occurring in the Millstone Grit. These acre- 

 spires are greyish or brownish in colour and very hard ; some are of immense size. 



Dolomite occurs as concretions in the compact maguesian limestone. 



Flint occurs as concretions in a variety of shapes in the chalk. 



Imatra Stones occur in a marly formation in Finland. 



Sphcerosiderite occurs as spherical masses on various iron ores. 



Clay Ironstone occurs as concretions in the sliales of the coal-measures ; these 

 concretions are round but flattened, and of a grey colour. This form of ironstone 

 is the ore used at Lowmoor in the smelting for iron. 



Limonite occurs in concretions in the chalk; some are formed by the oxidation 

 of nodules of iron pyrites. 



Iron Pyrites occiu' as concretions in the challc ; most of the concretions show a 

 radiated structure. 



O.iide of Manyane.se occurs as concretions on the bed of the Pacific. These 

 nodules are forming at the presejit time. The}' generally enclose some fragment of a 

 shell or bit of pumice as a nucleus. 



C'o;j/-oZj!:es are concretions occurring in the greensand and other formations; some 

 of them are the fossil dung of reptiles, but most of them are concretions. 



Pseudocojn-olites occur in the (Jrag ; they resemble coprolites in composition. 



As to the formation of these concretions little is at present known, and I wish 

 to draw attention to the manganese nodules which are at present forming, and 

 which, if examined, I am sure would throw light upon the subject. 



11. On a Scrohicularia Bed containing Human Bones, at Newton Ahhot, 

 Devonshire. By W. Pengellt, F.B.S., F.G.S.—See Section H, p. 841. 



Geology Sub-Section. 



1. The Gorndon Laccolites. By "W. W. Watts, M.A., F.G.S. 



The paper began with an account of two groups of ashes and lavas, of andesitic 

 composition, which had been mapped by the author and Professor Lapworth in 

 the neighbourhood of the Corndon Mountain, in Montgomeryshire. The lower of 

 the two groups is of Upper Arenig, while the upper is of Llandeilo-Bala age. Each 



