18 Missing Chapters of Geological History. 



In passing from Silurian to Devonian, there has always been 

 a difficulty among English geologists ; for we have the Old Red 

 Sandstone in one part of the country, composed of sandstones 

 and conglomerates, and, in another part, the Devonian shales. 

 These great differences of mineral composition would of them- 

 selves ensure a marked change of organisms, and it is also pro- 

 bable that the typical Old Red of Herefordshire, the Old Red 

 Sandstones of Scotland, and the beds believed to be contem- 

 poraneous in Devonshire, may belong to various parts of one 

 great period. On the whole, it would seem probable that, 

 during this long period of time, the sea-bottom in this part of 

 the world was undergoing depression ; but that this was accom- 

 panied by occasional elevation there can be little doubt, and 

 thus considerable gaps occur, though they do not amount to 

 physical breaks. 



Besides the evidence of a break between upper Silurian and 

 lower Devonian in England, we have in the south-west of Ire- 

 land, at the base of the Old Red Sandstone of that country, from 

 7000 to 10,000 feet of red beds, conformable to and overlying 

 Ludlow, or upper Silurian rocks, and above half the thickness 

 of other deposits, altogether unconformable, and passing up 

 into the Carboniferous limestone. There is no representation 

 of the deposits of the intermediate period in Ireland ; although 

 so great was the interval, that neither in fossils, nor in any 

 physical relation, is there any connection between them. The 

 Caithness flags in Scotland afford evidences of considerable 

 breaks of stratification ; but it is chiefly in the Pentland hills 

 that the proof is to be found. There would seem, on the whole, 

 no doubt that in all parts of the country where Devonian rocks 

 or Old Red Sandstone occur, there are two well marked breaks 

 at least between the commencement and close of the Devonian 

 period. 



Of the large number of species of upper Silurian fossils 

 now known (amounting to several hundred), less than ten have 

 been found in the lower Devonian, or immediately overlying 

 rocks. That this almost total difference corresponds to a lapse 

 of time during which the species gradually assumed new forms 

 adapted to altered conditions, is the view of those geologists 

 who endeavour to avoid the assumption of violent cataclysmic 

 action; and this view is not rendered less probable by the 

 mechanical and chemical changes that took place in the rocks 

 between the completion of the one and the commencement of 

 the other. 



The Devonian rocks of England are not, on the whole, rich 

 in fossils. The lower division (including the beds sometimes 

 regarded as middle) yield 170 species, referred to sixty-one 

 genera. Of these, twenty-three pass into the upper beds. These 



