this time a set of deposits _lmn_ may have been laid down elsewhere, and besides the gradual disappearance of some of the organisms of _ab ... k_, there will have been a gradual appearance of new species. When the upheaved area is once more submerged, a new set of deposits _op_ ... is accumulated in it, and the species which gradually appeared in adjoining regions will now migrate to it, and will seem to come in simultaneously at the bottom of _o_; accordingly we may find that there is not a single species which passes through from _f_ to _o_ and the palæontological break in this area is complete, though it is clear that it only implies local change, and that we may and indeed must find intermediate forms in other regions which fill up the gap. As an illustration of the local character of a palæontological break we may cite the case of the Carboniferous and Permian systems of Britain. These rocks are separated from one another in our area by a physical and palæontological break, but in parts of India, and other places, we find a group of rocks now known as the Permo-Carboniferous rocks which contain a fauna intermediate in character between those of the Permian and Carboniferous systems, and a study of this fauna shows that the hiatus which exists locally is filled by the species contained in the Permo-Carboniferous rocks. A palæontological break may, like a physical one, result from depression of the ocean-floor to so great a depth, that no organisms are preserved there during the period of great depression, and the remarks made concerning a depression of this nature when speaking of physical breaks will apply here also. A local palæontological break may result owing to physical changes without the production of an unconformity in the area, or its submergence to a great depth, or if an unconformity is found, the break may be more marked owing to other physical changes. The difference between the Upper and Lower Carboniferous faunas is very marked in England, where the Upper Carboniferous beds were deposited under physical conditions different from those of the Lower Carboniferous, and accordingly the corals, crinoids and other open-water animals which flourished in Lower Carboniferous times are rare or altogether absent in the higher rocks. Where the change of conditions did not occur to a great extent as in parts of Spain and North America, the similarity between the two faunas is much more pronounced. Again, there is an unconformity between the Cretaceous and Eocene beds of England, which is accompanied by a palæontological break, but this break is more pronounced owing to difference of physical conditions, for we find abundance of gastropods in the lower Tertiary beds, and a rarity of these shells at the top of the chalk of England, though where physical conditions were favourable for the growth of gastropods, their shells are found in the higher strata of chalk age, and the palæontological break is not so apparent.