538 J. Lonias — The Crystalline Gneisses. 



solidification by cooling of the earth ; and as having been formed in 

 a sea of high temperature, charged with chemically active substances 

 and subject to the frequent intervention of adjacent liquid magmas. 



The continuit}' of these ancient rocks and their persistent 

 characters certainly point to a cause of origin which was world- 

 wide in its effects rather than local. The main difficulty is to find 

 a worldwide cause which could produce the foliation so characteristic 

 of these rocks in all parts of the globe. 



Modern researches show that gneissose and schistose structures 

 can be induced either by fluxion or shearing, or by both combined. 



The ancient gneisses are usually found as low hummocky hills ; 

 when sufficiently denuded a core of granitoid rock is seen, and the 

 foliations of the flanking parts are parallel to the axes of the ridges. 

 Sir A. Geikie ^ remarks on the similarity of scenery which these 

 rocks show in places where they crop out. A person situated in 

 the central part of Anglesey might very well, so far as the landscape 

 is concerned, imagine himself in the wilds of Sutherlandshire. 



Professor G. H. Darwin^ has demonstrated that in early times the 

 moon was thrown off from the earth, and while it was still in 

 proximity to the parent planet great tidal waves would result from 

 its journey round the earth. The plane of the moon's orbit being 

 but little removed from the earth's equator, the retardation of the 

 diurnal motion of the earth, due to tidal friction, would be greater at 

 the equator than at the poles. "Now, this sort of motion, acting on 

 a mass which is not pterfectly homogeneous, would raise wrinkles on 



the surface In the case of the earth the wrinkles would 



run north and south at the equator, and would bear away to the 



eastward in northerly and southerly latitudes The 



general configuration of the continents (the large wrinkles) on the 

 earth's surface appears to me remarkable when viewed in connection 

 with these results." ^ These conclusions of Professor Darwin were 

 arrived at altogether independently of geological reasoning, and it 

 may be worth while to consider whether we have any geological 

 evidence to support his theory. 



The result of the moon's action on the earth in early times, then, 

 would be a screwing or twisting of the surface layers. This raised 

 them into wrinkles or ridges north and south at the equator, and 

 partaking more and more of an easterly direction as we approach the 

 poles. It is probable that at this time the earth was in a highly 

 heated condition, and thus flow and shear of the surface parts over 

 the lower would combine to produce the wrinkles. Probably the 

 wrinkles were formed very slowly, by the gradual accretion of 

 small stresses acting over long periods, in a manner so well 

 illustrated by our ex-President when speaking of the flow of rocks 

 by alternate expansion and contraction.*' 



1 " Ancient Volcanoes." 2 Phil. Trans. 1879, p. 589. 



^ Ibid. Also read in this connection, chap, ix, " Physics of the Earth's Crust," 

 0. Fisher. 



* "Origin of Mountain Eanges," by T. Mallard Eeade ; see also Geol. Mag. 

 1894, pp. 203-14. 



