Ch. XXVI.] FOSSIL FISH OF THE OLD FED SANDSTONE. 5 31 



system, and considered by him to be older than the division C, of the 

 table, p. 524, or those paving-stones and roofing-slates of. Forfarshire, 

 which contain Cephalaspis and Pterygotus before described, p. 526. 

 He fell naturally into this mistake by observing that the fish-beds 

 where he studied them most carefully, at Cromarty, were in almost 

 immediate juxtaposition with certain crystalline or metamorphic rocks, 

 so that they seemed to form the base of the Devonian system. 

 Another source of error, says Sir R. Murchison, arose from the grad- 

 ual thinning out of the bituminous and calcareous schists and flag 

 stones as we proceed from north to south. Already these schists, 

 when we reach Nairn and Elgin, are represented by clays with calcare- 

 ous nodules only ; and this is still more the case at Gamrie in Banff". 

 Still further southwards even these nodules are no longer traceable in 

 the middle portion of the Old Red Sandstone.* 



Hence the relative position of the middle and lower beds could not 

 be proved by direct superposition, the Caithness fish-beds being want- 

 ing in Forfarshire, and the Forfarshire Cephalaspis beds alike absent 

 in Caithness. But all doubt as to the true order of superposition, if any 

 still remained, was set at rest in 1861, when Mr. Peach, under the direc- 

 tion of Sir R. Murchison, searching for fossils in Caithness, found in 

 sandstones, many hundreds of feet below the fish-zone, undoubted re- 

 mains of Pterygotus. These crustaceans are characteristic of the Cepha- 

 laspis zone, and have never been found in the great fish-bed of the mid- 

 dle division of the Old Red. This discovery, therefore, confirmed the 

 anticipations of Sir Roderick, who had previously maintained that the 

 lower sandstones of Caithness were the equivalents of the Forfarshire 

 paving-stone, and of certain beds of Herefordshire and Shropshire, 

 which immediately overlie the bone-bed of the Upper Ludlow.f 



Mr. Powrie remarks that very few genera and no species of fish are 

 common to this Lower or Cephalaspis division, and to the Middle or 

 Caithness beds, whereas no such marked break occurs between the 

 ichthyic forms of the Middle and those of the Upper or Yellow Sand- 

 stone division.;]; 



Classification of the Fossil Fish of the Old Red Sandstone. 



The fish of the schists and flagstones in question are very peculiar 

 and characteristic. They were first successfully studied by the late 

 Hugh Miller, who gave an admirable description and restorations of 

 many of them. They were also the subject in 1844 of a special mono- 

 graph by Agassiz, in which he described no less than sixty-five British 

 species alone, and several important memoirs on Pterichthys and other 



* Murchison, Siluria, 3d ed., p. 286, 1859. 



f Powrie, Geol. Quart. Journ., vol. xiv. p. 503, 1858 ; and Murcnison, Siluna, 

 3d ed., p. 280, &c, 1859. 

 % Powrie, ibid., p. 428. 



