Page:VCH Cornwall 1.djvu/68

 A HISTORY OF CORNWALL apparent that the breccia has originated in little overfolds which have been isolated by small thrust faults. Although direct evidence of this origin is not always forthcoming, we occasionally see, step by step, every process in the manufacture, beginning with a regular banded rock, followed by minute folding and small thrust planes, partially severing the bands, until at last these processes have not only succeeded in brecciating the rock, but have culminated in the rolling and rounding of the frag- ments to such an extent that a pseudo-conglomerate has been produced. This type of brecciation has sometimes been brought about without folding ; instead of fractures succeeding plication, the stresses have re- sulted in the direct production of movement planes, which have isolated the fragments from the main mass. These pseudo-conglomerates are not confined to that restricted area in which they are so prominent, but are met with over many parts of the county. At Rosemullion a bed of this nature 2 or 3 feet in thick- ness bears a striking resemblance to a boulder bed ; the fragments, which may attain a length of 3 inches, are rounded, and consist of hard sandy material precisely similar to the matrix in which they are embedded. In the Veryan area this structure is noticed close to the junction of the slate and quartzite of that district. In one of the bands of brecciated slate we observed a crinoid stem, apparently unbroken, unmistakable evidence that the rock, as a whole, may undergo considerable deforma- tion, while portions may escape, probably as a result of the very violence of the processes which have produced zones of fracture relieving the strains on intermediate material. A brecciated structure of a much coarser type, which severs the continuity of thick seams of rock, occurs on the coast section south of the Helford river. Beyond that horizon the ' killas,' which consists of blue slates and fine sandy beds of the same type which occurs in Gerran's Bay and Falmouth Bay, has suffered more severely from the stresses which have induced the folding and the cleavage of the rocks. At Dennis Head the structures set up by these movements have been carried a step further. The strata have been subjected to crushing or mylonitiza- tion of a coarse type, insomuch that they have been reduced to a mass of coarse lenticular patches of rock, the lenticles being several feet, in some cases several yards, across, the whole presenting the character of a regional breccia. These lenticles, when they have been torn from quartzose beds, resemble huge boulders ; in other places the apex of a folded limb has been detached and isolated from the parent mass. This extremely coarse type of brecciated structure is more or less continuous along the coast as far as Porthalla, where the northern edge of the Lizard ' complex ' appears. The quartzite which occurs a little inland above the cliff, south of Nare Point, is represented on the shore by a lenticle about I o feet long and 5 or 6 feet wide, having been isolated by these move- ments from the parent mass. While earth stresses have so deformed 20