PROICHTHYAN PERIOD. 171 



PROICHTHYAN? PERIOD. It is to the sea that the oldest 

 trace of life which geology has discovered belongs. There is no 

 mark of the existence of land in any part of the area now occu- 

 pied by Yorkshire in all the palaeozoic period. It was then part 

 of the sea-bed, continually growing upward by additions of 

 argillaceous and arenaceous sediment, and the exuviae of fucoid 

 plants, Zoophyta, Mollusca, Annulosa, and Crustacea. Singular 

 to say, we have not yet found in the strata of this period any 

 sure traces of the race of fishes. This negative character may, 

 however, fail under further and more fortunate research. 



The monuments of this period which exist in Yorkshire are 

 the slaty and flaggy rocks of Hougill Fells, the slaty rocks of 

 Ingleton, and the flags of Bibblesdale. Of these, possibly the 

 greenish slate of Ingleton may be counted oldest, may be of the 

 same age as the slate of Coniston Fells, in which I have found 

 some traces of life. The flaggy series of Hougill and Ribbles- 

 dale may in a large sense be regarded as of like age, but the 

 group of strata in each is so thick, that the lowest part approaches 

 in age to the limestone of Coniston Water Head, and the upper 

 to the arenaceous and argillaceous beds some thousand feet above 

 it*. By Murchison they are called Lower Silurian, by Sedgwick 

 Upper Cambrian. Life-remains are plentiful in the Ribblesdale 

 flags, but difficult to extract in good condition. By the help of 

 Sedgwick' s last memoirf, and some notes of my own, it appears 

 that the earliest Yorkshire forms of life included only the natural 

 marine groups of Zoophyta, Brachiopoda, Cephalopoda, and Tri- 

 lobites, in all about seventeen species. 



PALICHTHYAN PERIOD. Of the calcareous, argillaceous and 

 arenaceous strata which, in a beautiful order of succession, are 

 enriched with multitudes of ancient forms of life, along the bor- 

 ders of Wales, there constituting the Upper Silurian series of 

 Murchison, we have in Yorkshire no ascertained trace. Had 

 the county now extended to its old Brigantian limit, had it 

 even stretched a few miles westward to the Lune, at Kirkby 



* Sedgwick, in Geol. Proceedings, 1851. f Ibid. 



