part 1] A>'XIYEESAKY ADDRESS OF THE PRESIDENT. lxv 



made, especially by those petrologists who urged the importance 

 of the dynamic factor in metamorphism ; but any systema tie- 

 study on genetic lines was still impossible, in default of fuller 

 physical and chemical knowledge. The situation is now changed, 

 and some petrologists, abroad and at home, have perceived that 

 this disability need no longer be admitted. 



Should this wider mode of vision become general, some arbitrary 

 limitations and inconsistencies which have hampered progress will 

 automatically disappear. By regarding metamorphism, not as a 

 peculiar state exemplified in certain groups of rocks, but as 

 a process or class of processes, we shall more definitely recognize 

 that it is something progressive, and, as estimated by its 

 results, a matter of degree. In particular, the earlier stages 

 or lower grades of metamorphism may be expected to throw 

 much light upon some important questions, and they undoubtedly 

 merit more attention than has hitherto been bestowed upon them. 

 In the purely descriptive treatment as commonly followed, the 

 rocks dealt with are such as have lost all trace of their former 

 mineralogical and textural characters, save only the occasional 

 recognition of residual or ' palimpsest ' structures. By excluding 

 the connecting links any determination of the original nature 

 of. the rocks metamorphosed is often rendered impossible, and 

 accordingly in a classificatory scheme such as that of Grubenmann 

 rocks of very different origin, one igneous and another sedi- 

 mentary, are grouped .together if, after total reconstitution. they 

 have a like chemical composition. 



It ma}" be remarked that, for the study of progressive meta- 

 morphism, British petrologists enjoy exceptional opportunities. 

 Owing to the dome-like disposition of the whole, the belt of 

 less metamorphosed rocks bordering a large metamorphic region 

 is of relatively narrow width. As a belt of weakness it is 

 liable to be cut out by later dislocations, and as the margin of a 

 mountain -tract it is likely to be concealed by the overlap of later 

 sediments. Gradations of metamorphism are in this way often 

 obscured, and the idea is fostered of some radical difference 

 between ' regional ' and ' local ' metamorphism. In the case of 

 the Scottish Highlands, however, the overthrusting and faulting 

 along the Highland Border have in great part spared the 

 marginal belt, and the Old Red Sandstone, which once spread 

 far up over the country, has been stripped away. In this element 

 of completeness, as in some other respects, the Highland area 



YOL. LXXIY. e 



