170 THE HABITAT OF THE EURYPTERIDA 



shire-Kincardine 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) west- 

 ern 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 classifica- 

 tions which have been made to fit not only the facts observed in the 

 field, but also the hypotheses evolved to account for the facts. More- 

 over, 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 overly- 

 ing 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 synchon- 

 ous 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 



