TI.] THE SLATES OX THE ROOF. 143 



For, throughout great tracts of Russia, and in parts 

 of Norway and Sweden, Sir Roderick Murchison dis- 

 covered our own Silurian beds, recognisable from their 

 peculiar fossils. But in what state ? Not contracted, 

 upheaved, and hardened to slates and grits, as they 

 are in Wales and elsewhere : but horizontal, unbroken, 

 and still soft, because undisturbed by volcanic rocks 

 and earthquakes. At the bottom of them all, near- 

 Petersburg, Sir Roderick found a shale of dried mud 

 (to quote his own words), " so soft and incoherent that 

 it is even used by sculptors for modelling, although it 

 underlies the great mass of fossil-bearing Silurian 

 rocks, and is, therefore, of the same age as the lower 

 crystalline hard slates of North Wales. So entirely 

 have most of these eldest rocks in Russia been ex- 

 empted from the influence of change, throughout 

 those enormous periods which have passed away since- 

 their accumulation." 



Among the many discoveries which science owes to 

 that illustrious veteran, I know none more valuable 

 for its bearing on the whole question of the making of 

 the earth-crust, than this one magnificent fact. 



But what a contrast between these Scandinavian 

 and Russian rocks and those of Britain ! Never ex- 

 ceeding, in Scandinavia, a thousand feet in thickness, 

 and lying usually horizontal, as they were first laid 

 down, they are swelled in Britain to a thickness of 

 thirty thousand feet, by intruded lavas and ashes ; 

 snapt, turned, set on end at every conceivable angle ; 

 shifted against each other to such an extent, that, to 

 give a single instance, in the Vale of Gwynnant, under 

 Snowdon, an immense wedge of porphyry has been 

 thrust up, in what is now the bottom of the valley, 



