Page:VCH Cornwall 1.djvu/69

 GEOLOGY the rocks as to interfere with the continuity of individual seams as in this quartzite, the same causes have in other cases produced quite an opposite effect ; for instance, the quartzite at Carne, which on the coast of Gerrans Bay below is only a few feet thick, has been so intensely folded that it occupies a considerable area on the hillside above, and presents the appearance of a bed of considerable thickness. In this case the quartzite has been folded, and the folds so packed together that a narrow bed has been made to occupy a considerable outcrop, while on the other hand the quartzite south of Nare Point (St. Keverne) has been so squeezed and dislocated that it is only represented by a series of lenticles. A small quartzite band that occurs in the Falmouth estuary presents similar features. On the coast between Restronguet Point and Porth- gwidden this quartzite seam i or 2 feet thick is represented by a series of lenticles among the folded and brecciated slates in which it occurs. The belt of country which is bounded by the coast line extending from Gerrans Bay to the Helford river includes so many estuaries, which penetrate far into the heart of the county, that the tortuous and ex- tensive coast line presents every facility for studying the rock structures. As this area includes several sedimentary divisions and every variety of deformation, a summary of its structural characteristics will convey a very fair idea of the processes of rock-building to which the Palaeozoic deposits of Cornwall have been subjected, as the result of their sojourn in the subterranean depths of the earth's crust. A detailed study of those coast sections reveals a complex set of structures, which have been brought about by powerful earth stresses. Folds may be detected immediately in any of the sedimentary divisions, but as the folds are generally isoclinal (closely packed) the plication is not so conspicuous as in a region of normal folding. But the evidence of plication is everywhere so marked that the apparent dip of the strata can only be interpreted as the inclination of limbs of folds. The folding has been accompanied by faults which are almost as numerous as the folds to which they bear a direct relation. The faults often occur every few yards, both parallel to the strike (or trend of the beds) and in oblique directions, with the result that the whole rock-mass when seen in ground plan presents the appearance of a huge mosaic. On that part of the coast with which we are now dealing neither large faults nor large folds are common. It would appear that at an early stage of the folding process the resistance to the strains was so feeble that fractures were readily produced. In many districts reversed faults are as numerous as those of normal type. Although the rocks are everywhere cleaved we can in most cases on the south coast distinguish between the cleavage and bedding. The alternations from argillaceous to arenaceous material are so numerous that there is little difficulty in distinguishing these structures. The cleavage which is usually oblique to the bedding varies with the nature of the 21