Page:The Habitat of the Eurypterida.djvu/178

170 and Perthshire area, with discontinuous outcrops in northern Argylshire, together with patches along the Caledonian Canal; (3) Scattered outcrops in southeast Scotland and the Cheviot Hills; (4) The southwestern and southern district of Wales; (5) western England together with the southern and southwestern portions of Ireland. The lack of geological and geographical continuity in these sections, the distinctness of the faunas where present, and the complicated tectonic relations, have led to many different classifications which have been made to fit not only the facts observed in the field, but also the hypotheses evolved to account for the facts. Moreover, since the deposits have not been formed in the sea, as I shall demonstrate below, none of the usual criteria for correlation of marine strata are available, and thus in each locality where the formations are described local names are given to the beds and it is impossible to state what are the equivalents elsewhere. The same lithological facies are repeated again and again, there being rapid vertical and lateral changes, but nowhere is the succession twice alike.

The original subdivision of the Old Red sandstone was made by Murchison before the middle of the last century into three groups, as follows:


 * Upper Old Red or Dura Den beds,
 * Middle Old Red or Caithness flags,
 * Lower Old Red or Arbroath flags.

The lower series is typically developed in Forfarshire, where it consists of coarse conglomerates for the most part, though shales and sandstone are also represented. The middle series is the remarkable grey, flaggy facies exhibited in Caithness and carrying the abundant fish fauna, while the upper is a yellow sandstone group found overlying the flags at Dura Den. Murchison's chief reason for making the Lower and Middle separate, even though the two are never found in contact or even in the same locality, was the distinctness of the faunas in the two, for while the fish and eurypterids of the Arbroath flags were generically and sometimes specifically like those in the Upper Siluric, they were entirely different from those in the Caithness flags, a statement which later investigators have strengthened. Geikie, however, contended that the Lower and Middle were synchonous deposits in separate lakes and that the faunas were not entirely distinct, and even today Geikie supports the two-fold division making the Lower include the Arbroath flags and the Caithness flags, while in