442 THICKlN^ESS OF SILURIAN STRATA. [Ch. XXYH. 



Fig. 599. 



~""" Fig. 600. 



^^ 



^ 



a, 5. Didymograpsus iGraptoliies) Mur- 



chisonii, Beck. 



Llandeilo Flags. Wales. 



Fi;;. COl. 



Didymograpsus geminus, Hisinger, sp. 

 Sweden. 



Fig. 603. 



Biplograpsus folium, 



Hisinger. 



Scotland: Sweden. 



Diplograpsus pristis, 

 Hisinger, sp. 



Shropshire; Wales; Sweden, 

 &c. 



Bastrites peregrmtis, Barrande. 

 Scotland; Bohemia; Saxony. 



Beneath the black slates above described no graptolites appear as yet 

 to have been found, but the characteristic shells and trilobites of the 

 Lower Silurian rocks are still traceable downwards, in ISTorth and South 

 Wales, through a vast depth of shaly beds, interstratified with trappean 

 formations, sometimes not less in their aggregate thickness than 11,000 

 feet. Hence the total thickness of the beds assigned to the Lower Si- 

 lurian, or the Llandeilo group of Murchison, is not less than 20,000 feet, 

 and the Upper Silurian rocks are above 5000 feet in addition. If these 

 beds were all exclusively of sedimentary origin we might w^ell expect, 

 from the analogy of other parts of the earth's crust, to find that they 

 must be referred paleontologically to more than one era ; in other words, 

 that changes in animal and vegetable life, as great as those which oc- 

 curred in the course of several such periods as the Devonian, Carbonifer- 

 ous, and Permian, would be found to have taken place while the accumu- 

 lation of so enormous a pile of rocks was effected. But in volcanic archi- 

 pelagoes, as in the Canaries for example, we see the most active of all 

 known causes, aqueous and igneous, simultaneously at work to produce 

 great results in a comi:)aratively moderate lapse of time. The outpour- 

 ing of repeated streams of lava, — the showering down upon land and 

 sea of volcanic ashes, — the sweeping seaward of loose sand and cinders, 

 or of rocks ground down to pebbles and sand, by torrents descending 

 steeply inclined channels, — the undermining and eating away of long 

 lines of sea-cliff exposed to the swell of a deep and open ocean, — above 

 all, the injection, both above and below the sea-level, of sheets of melted 

 matter between the lavas previously formed at the surface, — these op- 

 erations may combine to produce a considerable volume of superimposed 

 matter, without there being time for any extensive change of species. 



