of central and eastern cornwall. 493 



Conclusion. 



In addition to the rocks which occur in Western Cornwall, some 

 of the areas forming the subject of the present paper afford nume- 

 rous examples of ancient lava-flows so interbedded with the slates 

 and schists of the district as to lead irresistibly to the conclusion that 

 they are contemporaneous igneous products. 



These lavas, notwithstanding the alteration to which they have 

 been subjected, closely resemble those of more modern date, and 

 contain cavities which are now filled either with a mixture of viri- 

 dite and calcite or with quartz through which vermicular chlorite is 

 distributed. They sometimes assume a distinctly schistose structure, 

 and contain crystals of a sanidine-like felspar, many of which are 

 broken, while others have become sphgerulitic, through a rearrange- 

 ment of their constituents. 



The crystalline greenstones of Central and Eastern Cornwall afford 

 a more varied and instructive series than is furnished by those of 

 the western portion of the county. Among them are typical dolerites 

 on the one hand, as at South Petherwin, while on the other there 

 are rocks which, as at Carkeel, are so altered as to consist only of a 

 granular indefinite material traversed by indistinct microlitic bodies, 

 and, in patches, stained brown by oxide of iron. 



The pyroxenic constituent of these rocks, wherever its crystals 

 admit of identification, is augite ; and they are consequently dolerites. 

 By many petrographers they will doubtless be classed either as 

 diabases or melaphyres ; but when it is remembered that they differ 

 from modern dolerites only in age, and by reason of changes conse- 

 quent upon their greater antiquity, it would appear more logical to 

 call them ancient dolerites. 



Like their more modern prototypes, these rocks are often asso- 

 ciated with flows of vesicular lava, and have manifestly been pro- 

 duced by the operation of forces in all respects similar to those re- 

 sulting in the eruption of ordinary dolerites. If, therefore, distinct 

 names be applied to the ancient and modern varieties of this rock, 

 it would appear equally necessary to distinguish in the same way 

 between ancient and modern lavas. 



The changes which gradually take place in such rocks are usually 

 the transformation of augite into hornblende and viridite, while 

 the felspar becomes cloudy and finally merges into a granular 

 mass. The titanic iron is also more or less completely replaced by 

 a greyish- white product of alteration ; and a little epidote some- 

 times appears as a secondary product. Some of the grains of quartz 

 found in rocks of this description are probably the result of a dis- 

 sociation of silica. 



The transformation of augite into hornblende often begins with 

 an external hornblendic fringe, and the crystal finally becomes re- 

 placed by an assemblage of felted microlites. Less frequently this 

 change commences at one or more points within the crystal itself. 



In other cases crystals of augite are gradually replaced in such a 

 way that their outlines are distinctly and sharply preserved, while 



