48 



XII. 



CON'CRETIONS AND CONCRETIONARY ACTIONS. 

 Abstract of Fcuper 



BY 



Mr. ARTHUR S. REID, M.A., F.G.S. 

 Read before the East Kent Natural History Society, January 13th, 1889. 



The Author having distinguished between Concretions and Secretions. 

 divided the former into two classes : ( 1 ) those which have been formed 

 more or less contemporaneously with the beds in which they are 

 found : (2) those which have been formed subsequently to consolidation, 

 by segregation from the surrounding rock. 



In considering the forces at work to bring about concretionary 

 action, the Author described the probable way in which the phosphatic 

 nodules of the " Cambridge Greensand " were formed ; then passing to 

 concretions formed subsequently to consolidation, and in which there 

 was no trace of an organic "irritant," he pointed out the tendency of 

 particles of a mineral diffused in small quantities throughout a rock mass 

 to congregate towards definite points ; and discussed other similar 

 molecular actions especially in relation to " diffusion currents." 



The Author then described and exhibited specimens of the 

 more important forms of concretions ; peroxide of manganese con- 

 cretions of the deep sea were described, — and the probable origin of 

 chalk flints was pointed out. The solubility of organic silica in 

 carbonated water, and the replacement of carbonate of lime by silica 

 was demonstrated by the "green coated" flints of the Thanet-bed, 

 resting on tabular chalky flints now seen at the top of the chalk. The 

 origin and structure of the London Clay septaria was described, 

 and specimens of iron-sand concretions from the Old Red Sand- 

 stone of Scotland were exhibited, some of these shewing a con- 

 cretion in the process of making. The "half-hearted attempts" of 

 calcium sulphate to crystallize out in the Oldhaven sandy beds, was 

 contrasted with its more successful attempts in the finely divided London 

 Clay. Some remarkably globular concretions from the fine sand of the 

 Folkestone bed at Hothficld were exhibited and described. The foi-mation 

 of marcasite concretions in the chalk, &c., was fully discussed, and the 

 fantastic shapes assumed by the ''■ Loess-pitppen,^^ " Imatradeine,^' and 

 ^' Fairy stones,''^ touched upon, as also was "cone-in-cone" structure. 

 Finally, in relation to the symmetrical shapes assumed by some concretions, 

 it was pointed out that this might often be accounted for by the greater 

 freedom for molecular movements in rocks that were perfectly 

 homogeneous. 



The paper was illustrated by diagrams and specimens of concretions] 

 collected by the Author. 



