210 



KNOWLEDGE. 



[September 1, 1897. 



upheaval. The sea- worn terraces and raised beaches mark 

 the longer pauses in these movements. 



The depth of typical fjords often increases rapidly in 

 their inner reaches, the mouth being comparatively 

 shallow. This appears to be due to actual bending of the 

 original valley-floor — a phenomenon that has also con- 

 verted Alpine valleys into lakes ; but the peculiar narrow- 

 ness of fjords, and their freedom from detrital infiUings, 

 are usually ascribed to their having once been occupied by 

 glaciers. The river of ice, extending from wall to wall, 

 protects the valley sides from weathering, and carries ofl" 

 any stones and dust that may fall from the high peaks 

 above. At the same time, deltas and alluvial flats cannot 

 be formed upon the floor by stream-action. The glaciers 

 continued to occupy these valleys until after the land sank 

 beneath the sea, and the ravines were thus preserved in all 

 their original grandeur. When we pass outside latitudes 

 in which glaciation is a common feature, true fjords cease 

 to occur upon the continental edge. Submerged valleys 

 are, however, common, giving rise to inlets that clearly 

 were never excavated by the sea. Such inlets are usually 

 wider at their mouths than is the case in typical fjords, 

 and are shallowed in their inner reaches by sedimentary 

 deposits from the land. Von Eichthofen proposed to call 

 them " rias," from their occurrence vinder that name in 

 the Spanish province of Galicia (Fig. 8), and the term has 

 now become somewhat widely spread. It seems, however, 

 that ancient fjords must become worn away and filled up 



until they resemble "rias." 

 Suess and others have referred 

 the "fjords" of south-west 

 Ireland to the " ria " type; 

 but it is highly probable that 

 at one time they were occu- 

 pied by glaciers, and possessed 

 the characters of genuine 

 fjords. 



Perhaps we have said 

 enough to indicate the ten- 

 dencyof modern investigation 

 with regard to continental 

 edges, at any rate from a 

 stiuctural point of view. 

 The British Isles, standing 

 as they do on the European 

 plateau, but with their wes tern 

 Fio 3.— Coastline of Coruua edges close against the Atlan- 

 andPontevedra Galicia, slio«iDg tig depths, serve as excellent 



so-called "Bias. (Distance from „„,„'i „f .u _- i 



Eonh to south, oue hundred examples of the complex 

 jniles.) character of a coasthne. On 



the east, we have the delta- 

 deposits and smoother outlines against the shallows of the 

 North Sea ; and the east of Ireland shows similar leatures 

 towards the channel that separates it from England. While 

 the firths of Moray and of Forth in eastern Scotland indicate 

 subsidence, they remind us of the "ria" type, and the depth 

 within them rarely exceeds twenty-five fathoms. The 

 west coast of Scotland and of Ireland, and the south-west 

 coast of England, abound, however, in submerged valleys, 

 and show typical ijord-structure m the more northern 

 regions. We need only instance Loch Etive, Loch Fyne 

 (with its one hundred and four fathoms of water), and the 

 so-called Killary Bay. 



When we seek to correlate the continental edge of 

 Europe with the geological structure of our own islands, 

 we see that the hard and ancient masses of north-western 

 Ireland and of western Scotland, folded into ridges with a 

 north-east and south-west trend, continue the great Scan- 

 dinavian peninsula, of which they are obviously a part. 



Though the actual continental edge lies one hundred to 

 two hundred miles to the west of these Scotch and Irish 

 ridges, it seems natural to suppose that they ha%'e influenced 

 its direction as well as its position, if only by their 

 power of resistance. Farther south, however, as in Kerry, 

 the edge is at right angles to the direction of the ridges on 

 the land, and the fjords or " rias " creep up along the 

 trend of the folded Carboniferous and Devonian strata. It 

 is noteworthy that the last earth-folding of which we have 

 clear evidence in England — namely, that which drove the 

 Pliocene sea back from the Downs of Kent and Surrey — 

 gave us a ridge of chalk running east and west, in com- 

 plete independence of the edge of the continental plateau. 

 That edge, however, may have been farther to the west in 

 Pliocene times, and a good deal of oscillation has doubtless 

 taken place along it since it was established in its present 

 situation. The supposed fixity of ocean-boundaries, which 

 at one time threatened to become a dogma among 

 geologists, has ceased to confine our speculations. At the 

 same time, our duty lies in patient observation. The 

 study of folded strata in a small cliff-section may aid us in 

 the more difficult task of investigating a mountain-chain ; 

 and our mountain-chains, with their vertical and hori- 

 zontal movements, may in time allow us to correctly 

 appreciate the complexity of a continental border-line. 



ILttters. 



— » 



[The Editors do not hold themselves responsible for the opinions or 

 statements of correspondents.] 



WIND AS AN AID TO PLIGHT. 

 To tlw Editors of Knowledge. 



Sirs,— Mr. Headley's article in your issue for August 

 contains a startling dynamical theory which appears to 

 me to be based upon a misapprehension. I am not anxious 

 to cavil unnecessarily, and so I pass over his confusion of 

 the terms inertia and momentum— -^laxoh he uses indifferently 

 and apparently considers synonymous — and will deal with 

 the article as if he had used only the latter term. 



My quarrel is not, of course, with the statement that a 

 breeze will help a bird to rise. The fact in that statement 

 is almost as obvious as the fact that air, still or moving, is 

 necessary to "flight" ; what excites surprise is the stite- 

 ment that " inequalities of the whid " can give momentum 

 upwards to a bird, or to anything else for that matter. 



Mr. Headley tells us that " a uniform horizontal breeze 

 cannot lift or maintain a heavy body in the air unless that 

 body has momentum of its own." Very true ! but surely 

 not very new ! But then he goes on to imply that a wind 

 that 13 " unequal or not uniform " can lift and maintain 

 a heavy body when that body has no momentum of its 

 own, and this seems a baseless and quite visionary theory. 

 If it were true it would certainly be new, and the laws of 

 dynamics would have to be revised. But, surely, it is not 

 asking too much when I wish for some proof of the truth 

 of this novel theory before I accept it as a natural law. 

 As far as the article goes, we have nothing further in the 

 way of proof than a reiteration of the statement in 

 diflerent words. " When a bird rises," says Mr. Headley, 

 "with wings outspread and motionless, there must be 

 inequality, supposing the wind is horizontal." But that 

 is mere repetition of the unverified hypothesis. Would it 

 not be more in accord with what we know of the laws of 

 motion to say : " When a bird rises with wings outspread 

 and motionless through a horizontal wind, there must be 

 a momentum in the mass of his body acquired inde- 

 pendently of the layer of wind through which he is rising. 

 When that momentum is exhausted the horizontal wind 

 will not counteract gravity, and the bird with motionless 



