A HISTORY OF CORNWALL Before describing the various formations which compose the Palaeo- zoic system of Cornwall it will be convenient to give a brief description of the general structural features, which have imposed on them their present aspect. This will be more appropriate at this juncture as the geological phenomena to be described are posterior to the formation of the youngest member of the series ; and the chronological sequence we have adopted in tracing the course of our history from the events of to-day back to the remote past will not be departed from. From the description of the more recent deposits which we have already given it will be seen that the submarine platform from which Cornwall stands out has been subjected in the past to repeated oscilla- tions, so that the sea-floor of one epoch has formed the dry land of another, moreover the rocky platform of Cornwall itself constitutes, as regards large areas, ancient sea-floors which have emerged from beneath the waves. By these undulatory movements the adjacent bound- aries of land and sea have ever been shifting their position. When it is considered that the Pliocene shore line has been found at an elevation of 420 feet, and that the period which separates us from that landmark, in spite of its longevity, is but a fraction of the profound span which extends back to Palaeozoic times, it will require no strain on the imagi- nation to realize that the sediments accumulated beneath the seas of the more distant epoch form the main ingredients which enter into the composition of the Cornish platform. Although these Palaeozoic sediments present but little resemblance to the more recent deposits which rest upon them, and to the detritus now being accumulated on our littoral fringe, they are nevertheless to a great extent their counterparts, laid down in bygone geological periods, and thereafter subjected to a series of vicissitudes that have brought about metamorphic changes in their character. The 'killas' of Corn- wall, in spite of superinduced structures which largely mask its original condition, is seen to contain alternating layers of gravel, sand and mud similar to those which are observed in our recent deposits. Moreover, preserved within the strata are the fossil remains of the marine life that flourished in those ancient seas, but of types essentially different from our modern fauna ; while beds of limestone charged with fossils are the result of organic accumulations in clear water free from the inva- sion of sediment. In other words the rocks which enter so largely into the geological architecture of Cornwall belong to an early chapter in the history of that marine plain or great littoral fringe when its boundaries differed widely from those of to-day. Commingled however with the sediments we find the products of volcanic action in the remains of lava streams and ashes, which having been emitted on the sea-floor have been sealed beneath the overlying accumulation. Under the influence of crustal movements the Palaeozoic deposits with their volcanic beds have gradually subsided, and have been buried beneath a deep pile of overlying strata. So great has been their downward movement that they have been brought within the influence of the great laboratory that lies 16
