Geological Palaeontology

Foraminifera and such other small fossils as Ostracoda have recently attained a new and special importance in stratigraphical geology, because they can be determined in the fragments fetched up from bore-holes made with a jumper. The exploratory bores made very extensively to prove oil-fields yield such material, the investigation of which has become a special branch of palae ontology now actively pursued because of its economic importance, especially by American workers.

Establishment of Zones.

The zones so far considered rest entirely on an observational basis. They have been defined by recording the observed distribution of particular species in a series of rocks whose succession can be determined by mapping. It is clear that their validity depends entirely on the certainty with which the individuals of a species can be recognised and on the extent to which collections have been made. The business of a geologist intent on establishing zones is to collect as many fossils as possible from each bed of a well-exposed series, and then in the presence of the whole material to draw the limit of his species as finely as he can. That is, to indicate by a name (to be regarded purely as a method of handling the facts) each type of organism which can be distinguished from all others by observable and con stant morphological characters. Theoretically such groupings should be made without reference to the beds in which the indi vidual fossils have occurred, but in practice it is usually found that recognition of the more minute differences in structure is rendered much easier if the relative ages of the specimens under consideration be taken into account. Nevertheless, this practice, universally and necessarily pursued, introduces the danger that the optimism of the student may lead him into the acceptance as permanent and characteristic, of features which are not such in fact.

For full use to be made of the information acquired by the process suggested in the preceding paragraph, it is necessary to compare the species of fossils which have been thus recognised, with materials from other parts of the work,. As very few museums possess collections from all over the world and as these will in any case be inadequate, such a comparison can only be made with fossils previously described. It is therefore a primary duty of a palaeontologist to publish detailed descriptions :I! .s trated by accurate and clear figures of the forms he recognises, but he may only do so after he has compared his own specimens with the whole of the published accounts of similar creatures.

This process is extraordinarily laborious and tedious but is ren dered far easier if there is a modern monograph of the whole group involved. Af ter each individual fossil of the collection has been referred to a definite species, it is easy to discover whether any of them are restricted to a definite bed or series of beds, and it is then necessary to consider whether the absence of a given species at any particular horizon is due to the age of those rocks, or can be accounted for by a change of facies, which will in most cases be indicated by a change in the lithology. Life-zones are intended to be founded on the period of existence of a definite species of animal, and hence should have a validity throughout the entire region in which the animal in question lived. Thus zones should not be established, although they may be tentatively put forward, until it can be shown that they are recognisable over some area which represents a fair sample of the probable space-range of the species.

As it is known from present-day conditions that no individual species of sedentary animal which lives in shallow seas, occurs uniformly distributed over any area even so small as the English Channel, it is clear that the non-occurrence of the zone-fossil at any place does not necessarily imply that deposits of its zone are absent there. Thus even in rocks not lithologically dissimilar, the same zone may contain different faunas in places only a few miles apart.

If the zonal fossil be a pelagic form whose remains only secure burial after they have sunk from the surface to the sea-bottom, then within wide limits its occurrence should be independent of the nature of the sediments, and its distribution, depending as it will do on currents which change from season to season, may be expected to be uniform when periods of time exceeding a few years, are considered. Thus the non-occurrence of a pelagic zonal fossil in an area within which its remains are found, assuming that the collections are adequate, is good evidence that sediments laid down during its zone, were either not deposited or have subse quently been removed. The very detailed zoning of Jurassic rocks which has been established in England by the work of S. S. Buck man and his successors has shown that such failures of sedimenta tion or local unconformities are exceedingly common in shallow water deposits, and that they may be entirely unrecognisable by ordinary stratigraphical methods.