508 



ME. E. EETLEY OX THE EOCKS 



chlorite, linioriite, kaolin, serpentine, &c. ? Some would say that 

 they result from the pressure-metamorphism of eruptive rocks. It 

 is true that, in certain cases, slates of the ordinary kind may be 

 found to graduate into mica-slates as they near an eruptive mass : 

 yet is this the only way in which mica-slates and schists have been 

 formed? Admit that it is, and we may at once grant that the 

 gneisses and schists of the Malverns are but metamorphosed sedi- 

 ments. It may be so : we have yet to settle the limits of meta- 

 morphism. On the other hand, it seems well also to suggest the 

 possibility that the materials resulting from the denudation of 

 eruptive rocks go somewhere and form something, and that the 

 resulting rock is consequently likely to resemble those from which 

 its materials were derived, rather than a slate or sandstone. I there- 

 fore venture the suggestion that the gneissic rocks of the Malvern 

 Hills may be composed of the detritus of eruptive rocks. It is even 

 possible that in an early stage of the earth's history there was little 

 save rocks of an eruptive nature for the denuding agents to work 

 upon, and, if such an assumption were true, these Archaean rocks 

 would claim a far greater antiquity than if they resulted from the 

 metamorphism of stratified deposits. 



Again, it is very difficult to say how far the movements which 

 these rock-masses have experienced may have influenced their litho- 

 logical structure, and whether such movements have resulted in any 

 of the effects attributed to shearing. That such action is implicated 

 to a certain extent in the production of their foliation is highly 

 probable ; but how we are to distinguish between the crushing and 

 rearrangement of crushed materials by lateral thrust and the ar- 

 rangement of detritus by a sorting process (which may take place 

 either in air, in the case of volcanic dust falling on land and rocks 

 disintegrated and recemented in situ, or in water, in the case of 

 volcanic dust and rock-detritus deposited in seas or lakes) it is very 

 difficult to say. In either case we have triturated rock-matter, in 

 which a banded arrangement of the constituents prevails. It must 

 be admitted, however, that in the rocks of the Malvern Hills the 

 foliation bears but little resemblance to the structure induced by 

 shearing, the crystals and crystalline grains seldom showing any 

 marked lenticular form, while there is but little resemblance, as a 

 rule, to that pseudo-fluxion structure, described by Lehmann * and 

 other observers, which is so characteristic of rocks which have been 

 modified by a creeping movement along structural planes. 



My conclusion is, that the rocks of the Malvern Hills represent part 

 of an old district consisting of plutonic and, possihly, volcanic rocks, 

 associated with tuffs, sedimentary rocks composed mainly or wholly of 

 eruptive materic.ds, and grits and sandstones. That the structural 

 planes in these rocks, sometimes certainly, at others possibly, indi- 

 cate planes of stratification, and that the foliation in many cases, if 

 not in all, denotes lamination, due to deposition either in water or 

 on land surfaces, probably more or less accentuated or altered by 



* 'Entstehung der altkryst. Schiefer.' Bonn, 1884. 



