THE PROGRESS OF GEOLOGY. 



vii 



that the lower Silurian fossils were the earliest created forms^ 

 and that this type prevailed during that vast succession of 

 time which was occupied in the accumulation of all the older 

 slaty rocks^ until the upper Silurian period, w^hen new crea- 

 tures^ were called into existence, and when the earlier forms 

 diminished, and were succeeded by a profusion of chambered 

 shells, which so abundantly characterise that epoch. 



This deduction appears to have been founded upon exten- 

 sive examinations by Professor Sedgwick in Cumberland, 

 where he has traced the same general division into Upper 

 and Low^er Silurian types/^ in strata equivalent to those of 

 tracts heretofore denominated " Cambrian,^^ and also by Mr. 

 J. Marshall and Mr. Mac Lauchlan, who have recognised the 

 same Silurian types in the so-called Cambrian rocks of North 

 Pembroke. 



Mr. IMurchison alludes to the establishment of this theory 

 of organic succession, in his late address to the Geological 

 Society of London, in very marked and beautiful language, 

 and rejoices that the British islands have afforded the means 

 of working out the question systematically. Ascending from 

 the lowest types, he says : The upper Silurian zone is one 

 of great distinctness in England, and in the Baltic, in the 

 northern provinces of Russia, and in North America ; the 

 Wenlock, Dudley, and Ludlow fossils having been abun- 

 dantly found in both hemispheres ; as soon, however, as we 

 have advanced through this zone, a new" era is announced by 

 the presence of the earliest vertebrata. The minute and 

 curious fishes of the uppermost bed of the Ludlow rock, are 

 the earliest precursors of many singular Icytholites w^hich 

 succeed in that enormous formation, termed from its mineral 

 character in Scotland, and parts of England, the Old Red 

 Sandstone.* But in this, as in nearly every other deposit. 



* Vide Miller's Old Red Sandstone, or New walks in an old field. 



