48 MR. H. B. STOCKS ON THE ORIGIN OF CERTAIN  [ Feb. 1902, 
these coneretions are always associated with shells in the shale or 
roof of the coal, and therefore he attributed their formation to 
calcium-carbonate being dissolved away from these shells by per- 
colating water, and the precipitation or aggregation of it in certain 
centres of the vegetable matter now forming the coal below. 
Sir Archibald Geikie! says that the concretionary structure 
‘may be part of the original sedimentation, or may be due to whe eos 
segregation from decomposition round a centre.’ 
David Page* observed that concretions are formed by a molecular 
ageregation distinct from crystallization. | 
Dana * gives the following details :— 
‘ Percolating waters, aided by the carbonic or humus acids which such waters 
are likely to contain, dissolve the grains:and deposit the material, in a drying 
time, around grains, or any small object, as a nucleus. In like manner, con- 
cretions of limonite and iron-carbonate are made, if any ferruginous grains 
or any decomposable iron-bearing mineral is present. Occasionally other 
materials make disseminated concretions. 
‘The form of the concretion is not owing to any.central control of the 
molecular disposition, but to the regular progress of the superficial accretion, 
and to the rate of supply of the mineral solution in vertical and horizontal 
directions, together with the shapes of the nuclei.’ 
The search that I have made for information upon this subject 
has not been further rewarded than by the extracts just quoted. 
With reference to the formation of calcium-carbonate, we shall 
recollect that this mineral is deposited by several processes which 
need not be enumerated, so well known are they ; but it is necessary 
to bear in mind that calcium-carbonate in solution in water con- 
taining carbonic acid is rarely deposited in a rock, except in caverns 
or veins where the carbonic acid can escape. Indeed, it can be 
abundantly demonstrated that percolating water has almost in- 
variably the opposite tendency, that is, to dissolve the calcium- 
carbonate out of rocks through which it is percolating. As the coal- 
be?'s are not found in caverns or in veins, percolating water may be 
‘\s.-issed from any suspicion of being the cause of their formation. 
en, again, we cannot assume that the whole of the vegetation | 
co. ‘sing this bed of coal was swept down by one flood, and there- 
upon covered immediately after by mud containing shells. We must 
take it that a bed of coal even only 1 foot thick required an appre- 
ciable time to form, and during that interval] the plants would be so 
badly decayed that the vegetable tissues would be to a large extent 
destroyed, or at any rate much deformed. The fossil wood shows 
very little evidence of such destruction or deformation; and it is 
far more probable that these concretions were forming upon the 
plants as they were deposited in still water, and before any 
accumulation of other material upon them had taken place. The 
spherical, or nearly spherical, form of these concretions is also, to my 
mind, conclusive evidence that they were not produced under the 
pressure of superincumbent strata. 
‘Textbook of Geology’ Ist ed. (1882) p. 487. 
] : 
2 «Textbook of Aivanced Geology’ 5th ed. (1872) p. 454. 
3 «Manual of Geology’ 4th ed. (i896) p. 139. 
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