2,20 



NATURE 



\_August I, T889 



enables us to avoid the consideration of those changes in forces 

 which are functions of the positions of their points of application, 

 it is to be regarded rather as an ingenious device, unnecessary 

 perhaps now that we have learned to distinguish power from work 

 — that is, the fluxion from the fluent — but very useful when the 

 ideas of power and work were confused together, as until lately 

 they have been. At present it would perhaps be better to 

 enunciate D'Alembert's principle as follows : — 



" The algebraical sum of the powers of the external forces 

 of a passive system is equal to the sum of the powers of the 

 resultant forces." A passive system, for any motion, being 

 defined as " one the sum of the powers of whose internal forces 

 for that motion, is zero," F. Guthrie. 



South African College, July 2, 



"The Theorem of the Bride." 



Dr. Allman would be doing a service if he could trace the 

 origin of this term (see Nature, July 25, p. 299). Its occur- 

 rence in the "Scholia" carries its use back to an early date, 

 but hardly far enough. K. T. 



RECENT RESEARCHES INTO THE ORIGIN 

 AND AGE OF THE HIGHLANDS OF SCOT- 

 LAND AND THE WEST OF IRELAND} 



II. 



III. — The Silurian Period. 



AFTER the long interval of time represented by the 

 elevation of the red sandstones into dry land, and 

 their entire removal from some places by denudation, the 

 north-west of Scotland, and probably a large tract lying 

 around it, sank under the sea. The depression seems to 

 have been slow and gradual, and to have continued until 

 the site of the Cambrian basins and of the surrounding 

 region was covered with a considerable depth of clear 

 open sea-water. The records of this subsidence are con- 

 tained in a series of strata having a total thickness of 

 somewhere about 2000 feet, and divisible into two chief 

 groups — a Lower, composed of quartzites, grits, and thin 

 conglomerate, about 500 feet in total depth, and an 

 Upper, consisting almost wholly of limestone. Perhaps 

 the most striking feature in this series of stratified rocks 

 is the abundance of their organic remains. The quartz- 

 ites are crowded with the tubes formed by sea-worms 

 when the material existed as soft white sand on the sea- 

 bottom. The limestones are made up of the remains of 

 calcareous organisms, among which the most conspicu- 

 ous that now remain are chambered shells and Gastero- 

 pods. Throughout these limestones, worm-casts are 

 present almost everywhere, and in such abundance as to 

 show, as Mr. Peach has pointed out, that " nearly every 

 particle of the calcareous mud must have passed through 

 the intestines of worms." A large collection of fossils 

 has been made by the Geological Survey from these 

 limestones, which, though not yet specifically determined, 

 amply confirm the original generalization of Salter, made 

 more than thirty years ago, that the aspect or facies of 

 organic remains in the limestones of the north-west of 

 Scotland resembles that of the older parts of the Lower 

 Silurian formations of Canada rather than that of the 

 corresponding rocks in Wales. So marked is the re- 

 semblance to the American type as to indicate that some 

 shore-line must once have stretched across the North 

 Atlantic, in order to afford a platform for the free migra- 

 tion of marine life between the two areas. The contrast 

 with the Welsh type has been explained by the probable 

 existence of some barrier that separated the sea-bed over 

 the north-west of Scotland from that of Southern Scot- 

 land, England, and Wales. That such a barrier existed 



' The Friday evening lecture delivered at the Royai Institution on June 7, 

 by Dr. Archibald Geikie, F. R.S. Continued from p. ^02. 



is tolerably certain, and I shall presently refer to some 

 indications of its probable position. At the same time 

 it may be open to question whether the Durness lime- 

 stones can be properly correlated as homotaxial equiva- 

 lents of any Lower Silurian rocks in Wales. My own 

 impression is that they may be older than the oldest 

 Arenig rocks, and may be equivalent to some part of the 

 "Primordial Silurian" or Cambrian series. This, how- 

 ever, is a question that must remain unsettled until a 

 thorough critical examination of the fossils has been 

 completed. 



The area within which these Silurian quartzites and 

 limestones can be certainly recognized forms a narrow 

 belt extending for about no miles along the north-west 

 coast of Scotland, from the northern coast of Sutherland 

 to the south of the Island of Skye. Throughout that 

 extent of ground the rocks exhibit remarkable persistence 

 in the character and thickness of their several subdivi- 

 sions, whence the inference may legitimately be drawn 

 that the area within which they are now visible forms 

 but a small part of the region over which they were 

 originally deposited. 



it was claimed by Murchison, and generally conceded 

 by geologists, that the quartzites and limestones of the 

 north-west pass upward into a younger series of schists, 

 representing metamorphosed sedimentary rocks. This 

 order of succession appeared to be established by the 

 evidence of many clear natural sections along the whole 

 tract from Durness to Skye. It was first adopted and 

 afterwards opposed by Nicol, who in his later papers 

 maintained that the supposed younger schists were 

 merely the old or Archaean gneiss brought up again by 

 great faults, and pushed over the younger formations. 

 But he failed to account for the striking difference in 

 petrographical character between the old gneiss and the 

 younger schists, and for the remarkable coincidence be- 

 tween the general dip of the latter and that of the Silurian 

 stratified rocks on which they seemed to rest conformably. 

 During the last ten years, various geologists have renewed 

 the investigation of the question, among whom I may 

 specially mention Dr. Hicks, Prof. Bonney, Dr. Callaway, 

 Prof. Lapworth, and the members of the Geological Sur- 

 vey, particularly Messrs. Peach, Home, and Clough. The 

 result of their labours has been, in the first place, the 

 discovery of one of the most complicated pieces of geo- 

 logical structure at present known in any country ; in the 

 second place, the abandonment of all further controversy, 

 and the attainment of complete harmony regarding the 

 order of geological succession in the North- West 

 Highlands. 



Murchison's view that there is a regular upward pas- 

 sage from the quartzites and limestones into the upper 

 schists is proved to be erroneous, while Nicol's contention 

 that the old gneiss is brought up again above the Silurian 

 rocks is found to be, so far, correct. But the structure is 

 now seen to be infinitely more complex than Nicol ima- 

 gined, while, on the other hand, Murchison's belief that 

 the younger schists were evidence of a gigantic meta- 

 morphism later than Lower Silurian time is undoubtedly 

 true, though in a sense very different from that in which 

 he looked at the question. 



Nowhere in the North-West Highlands can any rock be 

 seen resting in its original and natural position above the 

 limestones. The highest Mmestone of Durness is the 

 youngest rock of that region about the geological position 

 of which there is any certainty. At present we know 

 absolutely nothing of other sedimentary strata which 

 followed that limestone. That such strata continued to 

 be deposited is certain, for the changes which the quart- 

 zites and limestones have undergone could not have 

 taken place save under the pressure of a thick mass of 

 overlying material. But this superincumbent mass has 

 been entirely obliterated in the extraordinary series of 

 terrestrial movements which I have now to describe. 



