Page:VCH Cornwall 1.djvu/48

 A HISTORY OF CORNWALL hollowed out of a district mainly composed of slate, while the great promontories by which it is flanked are built up of hard igneous rock ; that of the Land's End being composed of a boss of granite, the outer limits of which approximately conform to the coastline, while that of the Lizard, which constitutes the opposite spur, consists of igneous rocks the dominant member of which is serpentine. The rela- tion of these hard and tough rocks to the softer slates is expressed not only in the configuration of cape and inlet but the elevation of the cliffs shols similar correspondence, the cliffs of Mounts Bay being dwarfed by the more imposing mural faces of the Land's End and Lizard pro- montories. The granite of the Land's End, presenting a vertical wall to the Atlantic, forms the most impressive scenery ; while the serpentine, which enters so largely into the geology of the Lizard, breaks up into the most picturesque coast, the charm of which is enhanced by the natural polish induced by the action of the waves on that singularly beautiful rock. , ,, Our Cornish rivers, although frequently winding along deep valleys, are small and of low gradient ; in fact their declivity is so slight that their erosive powers on the valleys they traverse is now insignificant, matter of common observation however that after rams they are not only swollen and more rapid, but the usually pellucid stream is turbid with the s^iment borne along in its current to the muddy flats of the tidal estuaries on which it is deposited ; the force of the current being then so materially checked that none but the very finest material is capable of being held in suspension to be carried to the deeper water of the sea If our rive: system therefore exerts but slight erosive powers on the drainage basins which it occupies, it is evident that it plays an important par Tn transporting the washings of the soil to the sea. That the amount of material thus removed is by no means unimportant is testified by the rapid silting up of the tidal estuaries. In our harbours dredging is necessary to counteract such shallowing, while in some of the narrower creeks the silting up process is so rapid that the fact is familiar dwellers upon their banks. At Restronguet ships could ride at anchor considerably farther up the creek within the memory of men living, while the mud-flats of the river Fal below Ardevora are creeping rapidly seaward at a rate which is apparent to observation. Tradition avers that this estuary formerly extended inland as far as Tregonev, which was an important Roman settlement accessible shipping. The Fal estuary is of especial interest as the tidal flats, instead of being banks of dark brown mud derived from the de- composition of the slates, consist of pale tenacious clay brought down from the surface of the western portion of the St. Austell granite mass. That mass being in a highly decomposed condition readily lends itself t the process of denudation, in striking contrast to the neighbouring slates, which in comparison disintegrate slowly, so that the material deposited in the estuary is made up mainly of impure kaolin from the deposits of that district, considerably augmented by the drainage