742 REPORT— 1893. 



been originally sands like those of Hampstead Heath by the presence in them of 

 narrow bands rich in zircon, rutile, and the other heavy minerals which are so 

 constantly present in the finer-grained arenaceous deposits of all ages. Such 

 pleasant surprises as the recognition of a character like this increase our confidence 

 in the theory which endeavours to explain the past by reference to the present, 

 . and refuses to admit the necessity of believing in the existence of rocks formed 

 under physical conditions different from those which now prevail simply because 

 there are some whose origin is still involved in mystery. 



A crystalline schist has been aptly compared to a palimpsest. Historical records 

 of priceless value have often been obscured by the superposition of later writings ; 

 so it is with the records of the rocks. In the case of the schists the original 

 characters have been so modified by folding, faulting, deformation, crystallisation, 

 and segregation that they have often become unrecognisable. But when the asso- 

 ciated rocks have the composition of sediments we need have no hesitation in 

 attributing the banded structure in some way to str.atification, provided we clearly 

 recognise that the order of succession and the relative thicknesses of the original 

 beds cannot be ascertained by applying the principles which are valid in compara- 

 tively undisturbed regions. 



In studying the crystalline schists nothing, perhaps, strikes one more forcibly 

 than the evidence of crystal-building in solid rocks. Cliiastolite, staurolite, auda- 

 lusite, garnet, albite, cordierite, micas of various kinds, and many other minerals 

 have clearly been developed without anything like fusion having taken place. 

 Traces of previous movements may not unfrequently be found in the arrangement 

 of the inclusions, while the minerals themselves show no signs of deformation. 

 Facts of this kind, when they occur, clearly indicate that the crystallisation was 

 subsequent to the mechanical action. Nevertheless, it is probable that both 

 phenomena were closely related, though not in all cases as cause and efiect. The 

 intrusion of large masses of plutonic rock often marks the close of a period of 

 folding. This is well illustrated by the relation of granite to the surrounding- 

 rocks in the Lake District, the Southern Uplands of Scotland, and the west of 

 England. Those of the two first-mentioned localities are post-Silurian and pre- 

 Carboniferous, those of the last-mentioned locality are post-Carboniferous and pre- 

 Permian ; one set followed the Caledonian ^ folding, the other set followed the 

 Hercynian folding. That the intrusion of these granites was subsequent to the 

 main movements which produced the folding and cleavage is proved by the fact 

 that the mechanical structures may often be recognised in the crystalline contact- 

 rocks, although the individual minerals have not been strained or broken. In 

 many other respects the rocks produced by so-called contact-metamorphism re- 

 semble those found in certain areas of crystalline schist. Many of the most 

 characteristic minerals are common to the two sets of rocks, and so also are many 

 structures. The cipolins and associated rocks of schistose regions have many 

 points of resemblance to the crj'stalliue limestones and ' kalksilicathornfels ' pro- 

 duced by contact-metamorphism. - 



These facts make it highly probable that, by studying the metamorphic action 

 surrounding plutonic masses, we may gain an insight into the causes which have 

 produced the crystalline schists of sedimentary origin ; just as, by studying the 

 intrusive masses themselves and noting the tendency to petrographical difi'erentia- 

 tion, especiall3' at the margins, we may gain an insight into the causes which 

 have produced the gneisses of igneous origin.^ In the districts to which reference 

 has been made the igneous material came from below into a region where the 

 rocks had been rendered tolerably rigid. Difierential movement was not taking 

 place in these rocks when the intrusion occurred. Consider what must happen if 

 the folding stresses operate on the zone separating the sedimentary rocks from the 



' This term is employed in the sense in which it is used by Suess and Bertrand. 



- H. Eosenbusch, ' Zur Aulfassung des Grundgebirges,' Neues Jahr. f. Miner., 

 Ed. II. 1889, p. 8. 



^ G. Barrow, ' On an Intrusion of Muscovite-biotite-gneiss in the South-eastern 

 Highlands of Scotland, &c.,' Quart. Journ. Geol. Soc, vol. xlix. (1893), p. 330. 



