in the Magnesian Limestone. 435 



- These beds consist of friable yellow marl, in which are imbedded 

 hard sphasroidal concretions often much flattened. Tlie concretions 

 consist of 85-95 per cent, of carbonate of lime, and 6 to 12 per cent. 

 of magnesia, while the matrix often contains as much as 50 parts of 

 magnesia on 100 of carbonate of lime. 



The beds are markedly stratified, the lines of bedding passing 

 uninterruptedly through matrix and concretions alike. (See PI. XIII.) 



This fact was first noticed by Prof. Sedgwick, and it is perhaps 

 the strongest argument in favour of what we may call the ' Segre- 

 gation ' theory. The advocates of the ' Stalactitic ' theory refer 

 these lines of stratification in the concretions to lines along which 

 the carbonate of lime spread out when dripping from above in 

 solution ; but this does not account for the individual lines of 

 bedding passing uninterruptedly from matrix to concretion, nor 

 does it account (if the formation is still supposed to be taking 

 place) for the fact that where the beds dip at a considerable angle, 

 the lines in the concretions are still parallel to the general bedding 

 of the rock, not parallel to the horizon, as would be the case if 

 they were being formed up to the present day by infiltration. 



These flattened concretions contain numerous small shells dis- 

 seminated through them, notably Pleuropterus costatus and Turbo 

 Jielicinus, whereas these appear to be altogether absent from the 

 matrix. This we should expect according to the ' Segregation ' 

 theory, the shells being the nucleus round which the carbonate of 

 lime segregated, and any fossils remaining in the matrix, which 

 is extremely porous, would, after the solidification of the bed, be 

 readily dissolved out by subsequent infiltration, whilst those em- 

 bedded in the concretions would be protected by their impervious 

 covering of hard limestone. 



These fossils are frequently found projecting from the sides of the 

 concretion along lines of bedding, in the same way that one-half of 

 a sponge spicule, or other organic body, is found embedded in a 

 flint, while the other half projects into the surrounding chalk. 



It is difficult to account for the presence of fossils in these 

 flattened concretions if we assume their stalactitic origin, as we 

 cannot suppose them to have been introduced in solution, and to 

 have reformed as fossils on the solidification of the concretion. 



Turning now to the chemical side of the question. It had been 

 hoped that a detailed chemical investigation of these beds would 

 have conclusively proved by which of the two processes under con- 

 sideration these concretions had been formed. For supposing the 

 average analysis of the beds containing concretions to have agreed 

 with the analysis of beds containing no concretions, we should have 

 been justified in concluding that the carbonate of lime, of which 

 the concretions are mainly composed, had not been introduced from 

 overlying beds in solution, but had been originally deposited as part 

 of the bed in which they are now found; but the complicated nature 

 and variable chemical composition of the different beds of the series 

 cause the evidence so afforded to be less certain than had been hoped, 

 although the results are by no means valueless in this connexion. 



