Page:VCH Cornwall 1.djvu/70

 A HISTORY OF CORNWALL material it traverses, and is always deflected in crossing from an argilla- ceous to an arenaceous bed, and vice versa. The slates on this part of the south coast have a prevailing south- easterly hade, both in bedding and cleavage. By bedding should be understood the lithological alternations due to original stratification, which in most cases along the coast section represent the limbs of folds. Where these are regular in their hade, and the cleavage is uniform, the section may present such an undisturbed appearance that the folding might easily be overlooked. Where the folding follows the normal type it can be readily detected, but the amount of compression of the strata is often far less than when the folding is isoclinal, and the evidence of this is not so apparent. The obliquity between the cleavage and bedding is not constant. As the cleavage and folding have both been brought about by the same agencies, we see every variation depending on the character and curve of the fold, and the obliquity of the cleavage to the bedding will often vary over the different portions of the fold. Although we may often see the cleavage transverse to the bedding, the general tendency has been for the beds to be thrown into a set of isoclinal folds with a definite general hade, and for the cleavage to cross somewhat obliquely at a low angle. The more the folds depart from this general hade, the greater will be the variations between cleavage and bedding. Besides cleavage the slates are traversed by other structural planes analo- gous to those found in schistose rocks, and this is especially noticeable where they are made up of alternations of varying material. For not only have these composite beds been more readily acted upon owing to the less uniform resistance they have offered to pressure, but their banded appearance reveals the structures which in slate of more homogeneous material cannot be so readily detected. On examination we often, as already noted, find the rock full of tiny folds and thrusts, the latter passing through the axes of the former, while the secondary cleavages are often as prominent as the dominant cleavage of the beds. While it will be readily understood that under the influence of pressure the sands and muds of the Palaeozoic seas have been indurated and changed to the condition which is generally known in Cornwall as ' killas ' which along the coastline presents a rocky shore in striking contrast to the material of the beaches which fringe it the rock which has been referred to as quartzite is so markedly different in character that a word or so is necessary in explanation. The sandstone beds within the killas are built up from the aggregation of quartz grains, together with felspathic, and even argillaceous material, while scales of mica from original deposition are commonly distributed. Instead of this hetero- geneous mixture the quartzite consists of quartz grains among which other ingredients are but sparingly distributed. In the depths of the earth this purely siliceous deposit has been welded by pressure into a rock of uniform type corresponding to the uniformity of its ingredients, 22