Page:VCH Cornwall 1.djvu/83

 GEOLOGY The greenstones therefore have a considerable range in chemical composition, and vary extremely in their mechanical deformation, which is more pronounced in the basic rocks. It may be inferred therefore that the basic have experienced a greater severity of earth stresses than the acid varieties and are consequently of higher antiquity. What period separated the greenstone intrusions from the granites is not evident ; but as the granites themselves were subjected to stress prior to final consolidation the whole of these igneous intrusions may have been derived from a common magma. In investigating this genetic relationship however we are confronted with the fact that while some of these rocks retain their original structures, in others they are so far obliterated as to render them useless for comparative study. In so far as the granites and elvans are concerned their affinities are so pronounced that their petrological relationship is obvious. In the case of the green- stones however, both basic and intermediate, we can no longer trust petrological comparisons, but fall back on geological considerations based on the nature and extent of their deformation and mineral alteration, taken in conjunction with their relations to the granite intrusions and to the earth stresses with which we suppose these latter to be linked. From this point of view the elvans and mica traps are the latest. While the former were intruded when a state of equilibrium had been established, the mica traps appear to have shared in some of the stresses from which the elvans have escaped. The prior granite intrusions con- solidated under conditions of stress which induced parallel structures on them. The acid type of greenstone, although demonstrably older than the granite, has suffered comparatively little from earth movements, while the basic forms exhibit variations from types in which the original structure is partially preserved, to others in which it is entirely obliterated. It is possible therefore that the members of this igneous assemblage may be inter-related and have originated from a common magma. This hypothesis is indeed supported by the fact that it would include a con- nected series of igneous rocks from basic to acid, which would also represent the order of intrusion. The ancient Palaeozoic ridge of which Cornwall forms a part owes its origin to those earth movements the effects of which have been de- scribed, and the final result of which has been to raise the products of the subterranean depths to the surface. From the fact that the deposits were not submerged below that part of the crust known as the zone of frac- ture, we may assume that the subsidence was confined to a maximum depth of 5 or 6 miles. We have seen that the ancient sedimentary formations have suffered profound alteration by which their original characters are more or less obliterated. In spite however of this meta- morphism we can to a large extent decipher their more ancient history. That history covers an enormous span, for notwithstanding a very general 35