Mr C. J. Dixon writes: J. K. Leggett et al .have made an interesting contribution of particular importance to economic geologists who are interested in the development of mineral deposits during geological time. The authors make passing reference to iron-stone, but this aspect of early Palaeozoic sedimentation might be further developed. The vagaries of economic change and technological development have rendered most of the early Palaeozoic iron deposits uneconomic but a fair bit is known about them from mining in the past. Marine haematite ironstonebeds, frequently oolitic, occur throughoutEurope and its adjoining areas (Zitzmann 1977) from Anglesey to Morocco, and from Asturias in Spain to Czecho-slovakia. In North America similar ironstones occur in a discontinuous belt along the Appalachian foreland from Wabana in Newfoundland to Birmingham in Alabama. They are of special interest to the study of ironstones because they occur in rock sequences which can be classified stratigraphically in detail, unlike the larger and more economically important ironstones of the middle Proterozoic; however, they are too early to be understood in the same way as those of the Mesozoic.

It is clear that organic processes play a major role in the deposition and diagenesis of iron in the sea and thus a relationship between their formation and the 0 and ā€˜Gā€™ states of Fischer is to be expected. The distribution of ironstones in space and time is probably controlled by the availability of the metal. If the mounting evidence for the evolution of the oxygen content of the atmosphere

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