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 



