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 



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