O'Reilly — On the Phosphorite Nodules of PodoUa. 221 



diate origin. Amongst those specimens are pieces of balls entirely 

 composed of iron pyrites : one of these pieces is distinctly marca- 

 site, or liver pyrites, the characteristics of which are so well known. 

 Moreover, while pyrites occur in nearly every rock, marcasite is 

 more particularly associated with marls and clay beds, and in this 

 respect the observation of Mr. 0. Mene (0. E. Ixiv. 867), cited by 

 Dana in his 5th edition, p. 800 — that the pyrites of unaltered 

 sedimentary beds is mostly marcasite, while that of metamorphio 

 rocks is ordinary pyrites — is interesting and to the point. If now 

 we consider a bed of clayslate, in which originally marcasite 

 nodules were developed, these, by subsequent changes of sea level, 

 superposition of chalk-marl beds, and slow infiltration of solutions 

 containing carbonic acid and other constituents taken from the 

 chalk marls into the underlying slate rock, would undergo trans- 

 formations which might result at a certain stage in calcite ; and 

 then the subsequent changes would be explainable in the way 

 pointed out by Mr. Schwackhofer. The examination of a nodule 

 of marcasite or liver pyrites undergoing decomposition points to 

 this solution, since it shows not merely a radiated fibrous structure 

 exactly as that of the ph osphorite balls, but also a change of volume 

 consequent on the formation of sulphate of iron. This change is an 

 increase ; the mineral is therefore rent, and presents clefts exactly 

 as those occurring at the centre of certain of the nodules of phos- 

 phorite. The hoUowness of these clefts, or the absence of con- 

 tained matter, is easily accounted for, since these clefts would be 

 filled by the iron salt, and this would be the last removed, if the 

 action proceeded from the periphery towards the centre, and would 

 either be replaced by another mineral, if the solution could give 

 anything, or would simply be removed by the passage of water. 

 There would not, therefore, have been contraction, but rather the 

 contrary increase of volume, which precisely rendered the mineral 

 more apt to give passage to subsequent infiltration and metamor- 

 phism. As to the series of changes having led up to apatite, it 

 is presumable that it was more complex than that assumed by 

 Schwackhofer, who starts with calcite nodules. 



Starting from marcasite nodules, the clefts would tend to show 

 that change to sulphate of iron took place first, and that it was 

 complete. It m.ust be borne in mind that these nodules, being 

 entirely enveloped by clay more or less plastic, were protected 



