Vol. 62.] AN UNCONFORMITY IN THE COAL-MEASURES. 543 



Turning now to the detailed table of analyses, it will be seen 

 that, while there is a general similarity in the composition of the 

 various samples, JsTos. I & II stand apart in some respect from 

 the others, and the differences are such as might be expected to 

 exist between small pebbles on the one hand, and large slabs and 

 the mother-rock upon the other. Thus the insoluble matter is 

 higher in the former case, due to mechanically-mixed sand ; sulphur 

 too is high, owing to the presence of pyritic nodules among the 

 pebbles. The phosphorus-content of the small pebbles is lower 

 than that of the Mussel-Band, from which it would appear that 

 some leaching-out of the phosphates took place during weathering. 

 In this connexion, it is noteworthy that the sandstone in which 

 the pebbles are embedded is richer in phosphorus than the over- 

 lying pebble-free beds (see postea, p. 546). The small pebbles 

 contain more ferric iron and less carbon-dioxide than the other 

 samples, and this is in harmony with the normal weathering of a 

 rock consisting largely of ferrous carbonate. Lime is variable, 

 though on the whole higher in the large slab and the Mussel-Band. 

 Since the lime is derived chiefly from the fossil shells, and since 

 the presence of these makes the rock less able to withstand disrupting 

 influences, it follows that the small pebbles will be mainly made 

 up of the more homogeneous unfossiliferous portions of the rock. 

 Magnesia is remarkably constant in all the samples. A similar 

 phenomenon has been noted in the case of the Cleveland Ironstone, 

 and has been held to be due to uniform dissemination of the 

 magnesia in the form of glauconite. 1 An estimate of the amount 

 of magnesia thus combined is readily obtained by calculating its 

 equivalent to the shortage of carbon-dioxide ; that is, the amount 

 of carbon-dioxide still lacking when the acidic oxides have been 

 all appropriated by the basic oxides. In the samples under dis- 

 cussion, it averages about 2 per cent. 



It is clear, from the foregoing remarks, that the chemical evidence 

 supports the view that the pebbles are derived from the Mussel- 

 Band. This is brought out more strikingly by the following method 

 of treatment : — 



If the pebbles are more or less weathered portions of the Mussel- 

 Band, which itself at its present outcrop is somewhat altered by 

 weathering, then it should be possible, knowing the proximate 

 composition of the rocks in question and the chemical changes 

 which take place during weathering, to calculate the composition 

 of the original rock from which both have been derived. 



The only chemical change on weathering which need be considered 

 in the present case, is the oxidation of ferrous carbonate to ferric 

 oxide with loss of carbon-dioxide. 



In order to determine the proximate composition of the rocks, 

 we shall assume that the phosphoric oxide exists in combination with 

 ferrous oxide as ferrous phosphate, and that the remainder of the 

 ferrous oxide is present as carbonate; that the manganese is present 



1 R. Tate & J. F. Blake, ' The Yorkshire Lias ' 1876, pp. 164 it teqq. 



