inland, as they are today, and might come to rest in the very muds in which the river organisms were buried. Likewise, in the low-lying portions of the flood plain near the sea, occasional high tides or inundations, through the wearing away of sand-bars, might allow the salt water to enter, carrying some marine shells into those regions. Then the fossil fauna would show a large number of forms belonging mainly to one phylum, the arthopods, and occasional single specimens of members from other phyla. If the opposite conditions prevailed, and the fragments of the arthropods or their exoskeletons were blown to sea, then the fossil fauna would reveal many marine organisms, complete and well preserved, from all or nearly all of the invertebrate phyla, and occasional fragments of another group of organisms which were not well preserved and whose occurrence in such surroundings seemed anomalous.
Brackish-Water and Estuarine Deposits and Faunas. It has been shown that at the present time there is no such thing as a brackish-water fauna made up of classes of organisms different from those found in neighboring marine and fresh waters. It might be extremely difficult to recognize from the sediments and fossils that any fauna had lived in brackish water, because unless the salinity had been reduced so much that it was nearly that of fresh water the fauna would not appear to be very different from a typical marine one, except that it would be dwarfed and would contain few species. An estuarine fauna would likewise be difficult to recognize from the fossils. These would, however, be likely to be fragmentary, even comminuted to microscopic size, and larger forms would be found only in the sands and coarser deposits along shore and not in the estuarine deposits proper. It has been seen that the conditions in an estuary are not favorable for supporting life. The tidal scour, the churning up of the water, keeping the sediments constantly in suspension, the sudden change in salinity twice every day, are environmental factors not at all conducive to attract marine animals which can find more stable and beneficial conditions along the coast on both sides of the estuary. Thus, we saw that in the Severn the organisms whose comminuted remains were found in the muds lived many miles away in the quieter waters north and south of the estuary. In the geologic column we shall probably rarely be able to recognize estuarine deposits from the faunas, but if at all it will be from the nature of the sediments, their lithological characters and sources.
