384 



NATURE 



{^Attgust 26, 1S80 



morphic rocks have been produced, exclusive of the metamorphism 

 of igneous rocl-;s, on which I will not enter. These may be 

 loolied for in every manual of geology, and usually they may be 

 found in them. 



As a general rule, metamorphic rocks are apt to be much con- 

 torted, not only on a large scale, but also that the individual 

 layers of mica quartz and felspar in gneiss are bent and folded 

 in a great number of minute convolutions, so small that they 

 may be counted by the hundred in a foot or two of rock. Such 

 metamorphic rocks are often associated with masses of granite 

 both in bosses and in interstratified beds or layers, and where 

 the metamorphism becomes extreme it is often impossible to 

 draw a boundary line between the gneiss and the granite ; while, 

 on the other hand, it is often impossible to draw any true 

 boundary between gneiss (or other metamorphic rocks) and the 

 ordinary strata that have undergone metamorphism. Under 

 these circumstances it is not surprising that when chemically 

 analysed there is often little difference in the constituents of the 

 unmetamorphosed and the metamorphosed rock. This is a 

 point of some importance in relation to the origin and non- 

 primitive character of gneisi and other varieties of foliated strata, 

 and also of some quartzites and crystalline limestones. 



I am aware that in North America formations consisting of 

 metamorphic rocks have been stated to exist of older date than 

 the Lanrentian gneiss, and under any circumstances it is obvious 

 that vast tracts of pre- Laurent ian land must have existed in all 

 regions, by the degradation of which, sediments were derived 

 wherewith to provide materials for the deposition of the originally 

 unaltered Lanrentian strata. In England, Wales, and Scotland 

 attempts have also been made to prove the presence of more 

 ancient foi-mations, but I do not consider the data provided 

 sufficient to warrant any such conclusion. In the Highlands of 

 Scotland, and in some of the Western Isles, there are gneissic 

 rocks of pre-Cambrian age, which, since they were first described 

 by Sir Roderick Murchison in the North-west Highlands, have 

 been, I think justly, considered to belong to the Lanrentian 

 series, unconformably imderlying Cambrian and Lower Silurian 

 rocks, and as yet there are no sufficient grounds for dissenting 

 from his conclusion that tliey form the oldest known rocks in 

 the British Islands. 



It is unnecessary here to discuss the theory of the causes that 

 produced the metamorphism of stratified rocks, and it may be 

 sufticient to say, that under the influence of deep undergroimd 

 heat, aided by moisture, sandstones have been converted into 

 quartzites, limestones have become crystalline, and in shaly, 

 slaty, and schistose rocks, under like circumstances, there is 

 little or no development of new material, but rather, in the 

 main, a re-arrangement of constituents according to their 

 chemical affinities in rudely crystalline layers, which have very 

 often been more or less developed in pre-existing planes of 

 bedding. The materials of the whole are approximately the 

 same as those of the unaltered rock, but have been re-arranged 

 in layers, for example, of quartz, felspar, and mica, or of 

 hornblende, &c-, while other minerals, such as schorl and 

 garnets, are of not infrequent occurrence. 



It has for years been an established fact that nearly the whole 

 of the mountain masses of the Highlands of Scotland (exclusive 

 of the Lanrentian, Cambrian, and Old Red Sandstone forma- 

 tions) mostly consist of gneissic rocl<s of many varieties, and of 

 quartzites and a few bands of crystalline limestone, which, from 

 the north shore to the edge of the Old Red Sandstone, are 

 repeated again and again in stratigraphical convolutions great 

 and small. Many large bosses, veins, and dykes of granite are 

 associated with these rocks, and, as already stated, it sometimes 

 happens that it is hard to draw a geological line between gi-anite 

 and gneiss and vice vcrsA. These rocks, once called Primary or 

 Primitive, were first proved by Sir Roderick Murchison to be of 

 Lower Silurian age, thus revolutionising the geology of nearly 

 one-half of Scotland. To the same age belongs by far the 

 greater part of the broad hilly region of the south of Scotland 

 that lies between St. Abb's Head on the east and the coast of 

 Ayrshire and Wigtonshire on the west. In the south-west part 

 of this district, several great masses of granite rise amid the 

 Lower Silurian rocks, which in their neighbourhood pass into 

 mica-schist, and even into fine-grained gneiss. 



In Cornwall the occurrence of Silurian rocks is now well 

 known. They are of metamorphic character, and partly asso- 

 ciated with granite; and at Start Point, in South Devonshire, 

 the Silurian strata have been metamorphosed into quartzites. 



In parts of the Cambrian areas, Silurian rocks in contact with 



granite have been changed into crystalline hornblendic gneiss , 

 and in Anglesey there are large tracts of presumed Cambrian 

 strata, great part of which have been metamorphosed into 

 chlorite and mica-schist and gneiss, and the same is partly the 

 case with the Lower Silurian rocks of the centre of the island, 

 where it is almost impossible to disentangle them from the 

 associated granite. 



In Ireland similar metamorphic rocks are common, and, on 

 the authority of Prof. Hull, who knows them well, the following 

 statements are founded :—" Metamorphism in Ireland has been 

 geographical and not stratigraphical, and seems to have ceased 

 before the Upper Silurian period. 



" The epoch of greatest metamorphism appears to have been 

 tliat which intervened between the close of the Lower Silurian 

 period and the commencement of the Upper Silurian, taking the 

 formations in ascending order. 



"It is as yet undecided whether Lanrentian rocks occur in 

 Ireland. There are rocks in north-west Mayo very like those in 

 Sutherlandshire, but if they are of Laurentian age they come 

 directly under the metamorphosed Lower Silurian rocks, and it 

 may be very difficult to separate them. 



' ' Cambrian purple and green grits are not metamorphosed in 

 the counties of Wicklow and Dublin, but the same beds at the 

 southern extremity of county Wexford, near Carn^ore Point, 

 have been metamorphosed into mica-schist and gneiss. 



" In the east of Ireland the Lower Silurian grits and slates 

 have not been metamorphosed, except where in proximity to 

 granite, into which they insensibly pass in the counties of 

 Wicklow, Dublin, Westmeath, Cavan, Longford, and Down ; 

 but in the west and north-west of Ireland they have been meta- 

 morphosed into several varieties of schists, hornblende-rock, and 

 gneiss, or foliated granite." 



It would be easy to multiply cases of the metamorphism of 

 Silurian rocks on the continent of Europe, as, for example, in 

 Scandinavia and in the Ural Mountains, where, according to 

 Murchison, "by following its masses upon their strike, we 

 are assured that the same zone which in one tract has a 

 mechanical aspect and is fossiliferous, graduates in another 

 parallel of latitude into a metamorphic crystalline condition, 

 whereby not only the organic remains, but even the original im- 

 press of sedimentary origin are to a great degree obliterated." 

 The same kind of phenomena are common in Canada and the 

 United States; and Medlicott and Blanford, in "The Geology 

 of India," have described the thorough metamorphism of Lower 

 Silurian strata into gneiss and syenitic and hornblende schists. 



In Britain none of the Upper Silurian rocks have undergone 

 any serious change beyond that of ordinary consolidation, but in 

 the Eastern Alps at Gratz, Sir Roderick Murchison has described 

 both Upper .Silurian and Devonian strata interstratified with 

 separate courses of metamorphic chloritic schist. 



Enough has now been said to prove the frequent occurrence of 

 metamorphic action among Cambrian and Lower and Upper 

 Silurian strata. 



If we now turn to the Devonian and Old Red Sandstone strata 

 of England and Scotland, we find that metamorphic action has 

 also been at work, but in a much smaller degree. In Cornwall 

 and Devon five great bosses of gi-anite stand out amid the strati- 

 fied Silurian, Devonian, and Carboniferous formations. Adjoin- 

 ing or near these bosses the late Sir Henry De la Beche remarks, 

 that " in numerous localities we find the coarser slates converted 

 into rocks resembling mica-slate and gneiss, a fact particularly 

 well exhibited in the neighbourhood of Meavy, on the south-east 

 of T-avistock," and "near Camelford we observed a fine arena- 

 ceous and micaceous granwacke turned into a rock resembling 

 mica-slate near the granite." Other cases are given by the same 

 author of slaty strata turned into mica-schist and gneiss in rocks 

 now generally considered to be of Devonian age. 



The Devonian rocks and Old Red Sandstone are of the same 

 geological age, though they were deposited under different con- 

 ditions, the first being of marine, and the latter of fresh-water 

 origin. The Old Red Sandstone of Wales, England, and Scot- 

 land has not, as far as I know, suffered any metamorphism, 

 excepting in one case in the north-east of Ayrshire, near the 

 sources of the Avon Water, where a large boss of granite rises 

 tlirough the sandstone, which all round has been rendered 

 crystalline with well-developed crystals of felspar. 



On the continent of Europe a broad area of Devonian strata 

 lies on both banks of the Rhine and the Moselle. Forty years 

 ago Sedgwick and Murchison described the crystalline quartzites, 

 chlorite, and micaceous slates of the Hundsruck and the Taunus, 



