Page:VCH Cornwall 1.djvu/63

 GEOLOGY of life that tenanted the seas and lands of those remote epochs, the records of our county are silent. Whether those deposits ever extended westward on to the Cornish platform can scarcely be conjectured. That their absence in Cornwall does not negative their former presence in the west, we have had recently a vivid object lesson in the Isle of Arran, where the preservation of Mesozoic fragments in the vent of an old Tertiary volcano constitutes the sole record of the ancient extension of Rhastic, Liassic, and Upper Cretaceous strata to the basin of the Clyde. But the rocks which enter into the geology of Cornwall differ from the later Mesozoic formations not only in antiquity but in character. The traveller who journeys by rail from London to the west passes over the Tertiary and Mesozoic formations, the character of which is admirably brought out by the various cuttings which dissect them. As far west as the river Teign in Devonshire the strata, in spite of the most marked variations, preserve a general uniform facies in striking contrast to the character of the cuttings seen for the remainder of the journey west of that river. The sections to the east reveal soft strata, succeeded by beds which, although more coherent, rarely present the same massive section as those met with west of that river, which are distinguished by their greater appearance of solidity. This change of strata is accompanied by the most marked change in the character of the scenery, the broad plains and more gentle undulations of the east being replaced by a bolder land- scape, which has given rise to an extensive system of deep valleys that have been spanned by the numerous viaducts which form such conspicu- ous objects of the Cornish railway. In that westward journey we have been passing progressively across the various geological formations that occupy the interval between the Pliocene deposits of Cornwall and the Pakeozoic formations upon which they are reposing ; commencing with the older Tertiaries of the London basin, succeeded by the Cretaceous, Jurassic, and Triassic formations, which are finally replaced in south Devon by the Permian, so well seen in the red sandstones and con- glomerates that form the cliffs at Teignmouth. Those red beds rest on the Palaeozoic formations, which extend into Cornwall and almost mono- polize the geology of the county ; and as remarked in an early part of this chapter these older rocks form the natural foundations on which the strata of central and eastern England have been laid down. The succes- sive deposits, originally horizontal, have been tilted by crustal movements, so that in our western journey we pass over the truncated edges of great piles of strata, getting gradually lower in the series, just as we might walk over the upper edges of a pile of inclined slabs. The peculiarities which have given the Palasozoic formations of south-western England their marked character, especially their indurated condition, were induced on these formations before the Mesozoic epoch, as the Triassic and Permian deposits which have been partly made up of their detritus amply testify.