26 



KNOWLEDGE 



[Febhuaky 1, 1808. 



has been, to say the least, intensified during alteration. 

 Modern observation in this matter has supported the views 

 of that master geologist, Charles Darwin, who opposed his 

 opinion to that of Sedgwick, Lyell, and most of the teachers 

 of his day.* 



The present tendency is to regard the ancient schists and 

 gneisses as a complex mass of formerly molten materials, 

 which have successively intruded through one another, and 

 which have been, as a whole, deformed and foliated by subse- 

 quent pressures. I Sir A. Geikie suggests that the " over- 

 lying graphite-schists, mica-schists, and limestones of the 

 Gairloch and Loch Carron may thus be surviving 

 fragments of the stratified crust into which these deep- 

 seated masses were intruded," the latter masses now 

 forming the Lewisian gneiss of Scotland. 



In almost every area of ancient gneissic and schistose 

 rocks, there is found a series of true sediments, deposited 

 across the worn-down edges of the foliation -planes, but 

 still earlier than the fauna known as Cambrian. Examples 

 are the Huronian de- 

 posits of North America, 

 and the little - altered 

 Torridon sandstones 

 that form the bulwark 

 of western Sutherland. 

 The occurrence of frag- 

 ments of the funda- 

 mental rocks in this 

 overlying series shows 

 that the essential struc- 

 tures of the old complex 

 gneissic group had been 

 impressed upon it long 

 before Cambrian times. 

 Prof. Bonney ] is so 

 struck by this fact that 

 he regards the banding 

 of the gneisses as due 

 to conditions which 

 have not repeated them- 

 selves since ordinary 

 sediments began to be 

 deposited upon the 

 globe. Whether we 



FiQ. 1.— Block ,,f One 



complete passage from sediments into schists, and from 

 schists into gneisses, and urged that gneiss was the ultimate 

 stage of the alteration of ordinary sediments. 



At other times the fundamental gneissic mass is found 

 to send ofi' dykes and veins into the overlying rocks, which 

 we have hitherto regarded as being far younger than the 

 gneiss. Sometimes these appearances may be due to the 

 intrusion of a granite through both series, its close 

 resemblance to the gneisses allowing it to lie among them 

 undetected. But another solution has been offered, which 

 presents us with a new aspect of the continental floor. 

 Mr. .Joseph Nolan, in 1879, suggested that granitic 

 intrusions might arise from the depression and remelting 

 of an ancient metamorphic series. This series would 

 remain for the most part " fundamental" ; but its offshoots 

 would, of course, be later in age — /.c, in date of consolida- 

 tion — than the rocks invaded by them. Prof. A. C. 

 Lawsont has attributed much of the structure of the 

 Laurentian gneisses of Canada to this second period of 

 flow, and has provided 

 us with excellent 

 photographs of gneiss 

 including fragments of 

 the overlying series. 

 Similar phenomena are 

 recorded by Dr.Gregory ; 

 at the junction between 

 what was regarded as 

 " fundamental gneiss" 

 and the schists of the 

 Western Alps ; and the 

 conclusion is arrived at 

 that these central 

 gneisses of the moun- 

 tain-chain are as recent 

 as Miocene and even 

 Pliocene times. M. 

 Jlichel-Levy,; as is now 

 well known, has proved 

 that the gneiss- granite 

 of Mont Blanc 



wentv centimetres long, from Co. Mayo, showing . • • ., .1 ■ f 



(i.) curving upper surface formeil bv fracture along a foliation-plane ; (ii!) dissimilar '•"ll^l^S in tne ScniStS 



materials in different bands, the lighter ones consisting of quartz and felspar, and Surrounding it ; SO that 



the darker ones being rich in dark mica ; (iii.) a lenticular mass at the righi-hand here again we fail to 



adopt his view, or the «'°<*. "'"' t'le darker layers (lowing round it. recognise the true con- 



more rigidly iiniformi- tinental floor in its new 



tarian one of Sir Archibald Geikie, we must see in the guise of an igneous invader. General McMahon, again, 

 complex floor of schists and gneisses the oldest rocks sees in the gneissose granite of the Him:ilayas a rock of late 



For our present 



accessible to us in the earth's crust, 

 purposes they are " fundamental." 



Yet the upper boundary of the fundamental gneiss 

 presents difficulties when it comes to be surveyed in detail. 

 At times, subsequent pressures have obliterated the 

 discordances between the gneissic surface and the over- 

 lying stratified deposits ; the great earth-mill has rolled 

 all these rocks out together, and has produced a community 

 of structure, and even an appearance of continuity.; So 

 that there is little wonder that the older geologists saw a 



* " Geological Observations on South America," Minerva Library 

 edition, pp. 439 and 440. 



t Compare Sir A. Geikie, " Ancient Volcanoes of the British 

 Isles," Vol. I., p. 117; and C. R. Van Hise, " North American Pre- 

 Cambrian Geology," SLvteenth Annual Report, U.S. Geol. Survey, 

 1895, p. 753. 



X "The Foundation-Stones of the Earth's Crust," Nature, Vol- 

 XXXIX. (1888), p. 92. Compare a very interesting paper on crystalline 

 gneisses, by J. Lomas, P.G.s., Oeol. Magazine, 1897, p. 537. 



§ See Van Hise, op. cit., pp. "30 and 752. 



Eocene age, and regards its foliation as the result of pressure 

 acting while it was still a viscid mass. It is doubtful, 

 indeed, if the gneissic cores of mountain -ranges ever 

 represent the oldest rocks of the chain. Probably they 

 have no age but that of the folding of the strata. The 

 complex arch of stratified rocks was formed, and fused 

 material (often derived from the continental floor) was 

 forced into it as it rose. 



* " Metamorphic and Intrusive Bocks of Tyrone," Oeol. Mag., 

 1879, p. 1.59. 



t " Geologv of the Rainv Lake Region," Geot. Snrv. of Canada. 

 Ami. Report,'lSb7, pp. 130.140. 



X " The Waldensian Gneisses and their Place in the Cottian 

 Sequence," Quart. Jouni. Geol.Soc, Vol. L., 1894, pp. 235, 261, 270, 

 and 273. 



§ Bull, lies Services ile la Carte gi'ol. de la France, No. 9 (1890). 

 See also Gregory, '' Geologv of Western Alps," Science Progress, 

 Vol. III., p. 169. 



II Proc. Oeol. Assoc, Vol. XIV. (189.5), p. 93, and Geol. Maq, 1897. 

 p. 304, etc. 



