Page:The Habitat of the Eurypterida.djvu/158

152 area of over a thousand square miles and yet it never exceeds and seldom reaches 1 foot in thickness.

Following the Upper Ludlow in England comes a series of formations of no very great thickness which has been subdivided into the Tilestones, Downton Castle sandstones, and Ledbury shales. Murchison applied the name "Tilestones" to the whole of the flaggy upper parts of the Ludlow, and since many of the beds are red he included them in the Old Red Sandstone. They were believed to mark a transition period between the Upper Siluric and the Lower Old Red, but to be more like the latter with which they were therefore classed. The Downton sandstones are a group of red, yellow and gray micaceous rocks from 80 to 100 feet thick, occurring in the neighborhood of Downton Castle, Herefordshire, and also supposed to mark a transition period. They are undoubtedly indicative of the regressive movement of the sea, which began in Lower Ludlow time in Scotland but which was not strongly felt in England till the end of the Upper Ludlow. Then in the Downtonian and other "passage beds" were washed into the deposits, terrestrial and lycopodious, vegetal remains, together with eurypterids, Ceratiocaris and vast numbers of Beyrichia kloedeni, together with Lingula cornea and Platyschisma helicites.

In Scotland all of the beds above the Upper Ludlow are called, by the Geological Survey of Great Britain, the "Downtonian." This series is to be looked upon as "stratigraphical equivalents of the Tilestones, Downton sandstones and Ledbury shales which, in Herefordshire, overlie the Upper Ludlow Rocks and have been classified as forming the highest subdivision of the Upper Silurian rocks" (215, 568). It is evident that such a usage of Downtonian will lead to endless confusion, for not a little misunderstanding has already arisen because some authors have placed the English Downton beds in the Lower Old Red, and others have used Downton and Passage Beds almost synonymously. If an attempt is made to use Downtonian in a comprehensive way, coordinate in importance with the terms Wenlock and Ludlow, then difficulties will arise and much circumlocution will be necessary to explain whether the Downton of England or the Downtonian of Scotland is meant and in correlation difficulties will come about because so many different deposits are known by the same name. And especially does it seem inadvisable to adopt a name which in England is used for a subdivision of the Ludlow, and make it in Scotland of the same rank as Ludlow. Therefore, the author