Page:VCH Cornwall 1.djvu/84

 A HISTORY OF CORNWALL uniformity which these deposits exhibit as a consequence of their subter- ' ranean treatment, the period occupied by their formation may exceed that vast interval which has subsequently elapsed. For the present we will leave out of consideration the rocks which occupy the Lizard peninsula, which will be treated later. Of the Palaeo- zoic deposits which enter into the rest of the county the Devonian for- mation undoubtedly takes the most prominent position. Occupying the greater part of Cornwall, it is flanked on the north by the overlying Culm Measures, and on the south by Ordovician rocks, between which are interposed some non-fossiliferous beds, possibly Silurian. As the metamorphism to which these formations have been subjected is more intense in the western portion of the county, the original structures have consequently been better preserved in the eastern part. This difference is expressed in the greater preservation of the original minerals such as augite in the greenstones of east Cornwall, in the identification in the same area of undoubted lava and ash beds, the absence of which in the western region may possibly be explained by their more complete defor- mation having led to their inclusion with the intrusive greenstones. As regards the killas it may be stated that whereas in the western region the mechanical deformation has reached such an advanced stage that the rock structures are analogous to those of schistose rocks ; in the eastern area the process has generally stopped far short of that stage, and the cleavage has been sufficiently uniform to admit of the rock being exten- sively wrought for slate as in the well known quarries of Delabole. Moreover these differences in metamorphism are accompanied by the most marked divergence in the preservation of organic remains ; while the eastern area has yielded fossils in tolerable abundance, albeit in a poor state of preservation, the western district is characterized by their extreme rarity. The unravelling of the stratigraphical sequence therefore is attended with serious difficulties. For not only have the ancient stratigraphical boundaries been masked by the extensive deformation to which the rocks have been subjected, but their included fossils to which we might other- wise appeal as chronological landmarks have shared in these processes. The better preservation of the fossils in the eastern area has permitted the historical succession of the formations to be more accurately defined than in the west. An inspection of the map will show the southern boundary of the Culm Measures as extending from the vicinity of Boscastle to Horsebridge, which spans the Tamar near the horizon of Tavistock. The Devonian formation which occurs below the Culm Measures, although tolerably defined in its upper limits by that undulatory boun- dary, does not admit of such precise definition as regards its base. A zone that traverses the county from St. Austell Bay to Holywell Bay, approximately defines the limits of the lowest Devonian beds which have yielded reliable zonal fossils. South of that zone the killas is singularly barren of fossil remains 36