THE GEOLOGY OF THE YORKSHIRE JURASSIC FLORA

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Abstract
Summary: The stratigraphic relations of the flora of the Deltaic rocks of the “Yorkshire Estuarine Series” have remained unsatisfactory because of badly localised collections, ill-defined species and especially because of concentration of effort on the half-dozen best localities. A recent search over nearly the whole outcrop has given several hundred new localities yielding determinable plants (but mostly as small fragments). These show that most species continue through all four divisions of the series. There are however real differences in the floras of successive stages. One group of species is found in a good proportion of the hundred or so localities of the Lower Deltaic, but in the two divisions of the Middle Deltaic it has very few indeed of the numerous available localities and then has a good proportion of localities again in the Upper Deltaic. A contrasted group has numerous localities in the two middle divisions, but is rare in the first and last. This is more like a change in abundance caused by a fluctuation in climate than true zonation. It is to be noted that it is only the proportion of localities where a species is met that changes, not its abundance in a particular locality. Each locality has a flora of special character with a great abundance of certain species, and rarity of others. Even the rarest species may occur locally in millions of specimens and even form a tiny coal seam by itself.