544 PEOF. G. A. LEBOUR AND BR. J. A. SMYTHE ON [Aug. I906, 



as carbonate, since that body is not so easily oxidized as ferrous car- 

 bonate 1 ; that the lime exists as carbonate, and also that portion of the 

 magnesia for which there is still carbon-dioxide to combine with — 

 the remainder of the magnesia we have already assumed to be 

 present as glauconite. The following table gives the proximate 

 composition of the samples (recalculated for the dry rock). Under 

 the head of ' indifferent constituents ' are included ferrous 

 phosphate, combined water, organic matter, insoluble matter, 

 alumina, sulphur, magnesia (present as glauconite), manganese- 

 carbonate, and alkalies ; that is, those bodies which would be practi- 

 cally unaltered by such gentle and partial oxidation as takes place 

 during weathering. 



Proximate Composition of the Samples. 



I. II. III. IV. V. 



FeC0 3 57'71 5994 6212 51-50 65-20 



Fe 2 3 1086 6-45 1'36 183 0-32 



a^ioo ] 7 ' 64 12 " 79 15 ' 22 24 ' 93 16 " 53 



'Indift'erent constituents' ... 23-79 20-82 21*30 21 '74 17-95 



From this table we can now ' reconstruct,' as it were, the original 

 rock from which the various samples have been derived by 

 weathering. To do this, it is only necessary to replace the ferric 

 oxide by its equivalent of ferrous carbonate, and express the com- 

 position of the ' reconstructed ' rock in percentages. Thus we get 

 the following table, showing the composition of the ' reconstructed ' 

 rocks : — 



I. II. III. IV. V. 



FeC0 3 70-00 67-36 6373 53'74 65'57 



CaCO, 



7-30 12-43 15-13 24-74 1651 



'Indifferent constituents'... 22-70 20-21 2144 2152 17*92 



Now, it will be noted that the amount of the ' indifferent con- 

 stituents ' is approximately constant in the five samples, and that 

 the percentages of calcium- and magnesium- carbonates vary 

 inversely as those of the ferrous carbonate. The reconstructed 

 rocks, therefore, differ almost solely, owing to the greater or less 

 replacement of ferrous carbonate by the fossil-making carbonates 

 of calcium and magnesium. Thus, the chemical evidence points to 

 the fact that the Mussel-Band and the pebbles are derived from the 

 same source : the observed differences in composition being due, 

 firstly, to the varying amounts of calcium- and magnesium-carbonates 

 (the result of the irregular distribution of the fossil shells) ; and, 

 secondly, to the degree of oxidation by weathering which the various 

 samples have undergone. 



The appended diagram (fig. 13, p. 545), in which the ordinates 

 represent the composition of the ' reconstructed ' rocks, while the 



1 B. Brauns, ' Chemisehe Mineralogie ' 1890, p. 354. 



