May 1, 1899.] 



KNOWLEDGE 



113 



M' 



Table- Talk with Young Men. By W. J. Dawson. (Hodder & 

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The Siinferinn Oration, 1S99. By Sir William MaeConnac, Bart. 

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Ifotes upon the Soma no- British Settlement at Cliigirell, Etsex. 

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A Select Bibliography of Chemistry ~ 1492-1S97 . By Henry 

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Bulletin de la Societe Neuchateloise de Geographic. Tome XI., 

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Besults of Meteorological and Magnetical Observations, 1S98. 

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Seport of the TJ. S. National Museum for the Tear ending June 

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♦ ■ 



WHAT IS A GEOLOGICAL CATASTROPHE? 



By N. A. Gbaydon, 



'OST geologists of the present day take the 

 uniformitarian point of view, according to 

 which the general features of the earth's surface 

 are the product of, not sudden catastrophes, but 

 gradual and uniform changes, such as are now 

 more or less perceptibly in progress. There are, however, 

 some special features, the explanation of which seems to 

 demand changes so great and rapid or violent as to make 

 them hardly distinguishable from catastrophes. 



Generally speaking, disturbance of conformable strata, 

 such as upheaval, subsidence, tilting, folding, faulting, 

 erosion, etc., may well be conceived as a slow and 

 uniform process, but formation of conformable strata, 

 especially if of different nature, like, for instance, sand- 

 stone and limestone, is not easily representable as such ; 

 for the sudden transition from the last deposit of the one 

 kind of sediment to the first deposit of the other kmd 

 seems inexplicable, without a correspondingly sudden 

 change of level within or without the affected area. 



So long as there is no evidence in favour of great and 

 rapid changes of level, it may be right to assume their 

 non-occurrence ; but it is, in any case, advisable to 

 reconsider the available evidence on the subject in aU its 

 bearings, for some data, such as the preservation, more or 

 less intact, of submerged river valleys in the so-called 

 continental platform, seem to indicate immersion so 

 rapid as to leave no time for the action of the sea's 

 levelling agency. 



Under the title of " Sub-Oceanic Terraces and River 

 Valleys of the Bay of Biscay," Prof. Hull, f.r.s., gave in 

 Nature, of April 21st, 1898, some details of a typical case. 

 He says : " The Adour passes out to the deep ocean 

 through a continuous deep canon or gorge one hundred 

 miles long as 'Fosse de Cap Breton.' At thirty miles from 

 the coast the bed of the canon is three thousand one 

 hundred and seventy-four feet below the general level of 

 the continental shelf. At sixty-two miles it is about five 

 thousand four hundred and forty-two feet below the same 

 level. The cauon can be distinctly traced to the depth of 

 nine thousand feet where it opens out on the oceanic floor." 

 Accepting this account as correct, the question suggests 

 itself; How is it that this narrow submarine channel has 

 not long ago been filled up with river deposits or other 

 sediments travelling to and fro along the coast under the 

 influence of waves and currents, the action of which is 

 pretty well known and may be briefly summarized as 

 follows ; — 



I. Motion of the sea caused by the winds does not extend 

 far down. At a depth of thirty to fifty fathoms, wave 

 action is almost imperceptible and current scour ceases. 



II. Motion of the sea caused by tidal action is, in deep 

 water, chiefly vertical ; it is only converted into horizontal 

 motion in shallow water ; tidal currents are, therefore, 

 essentially surface phenomena and their scouring action 

 generally ceases at less depth than wave action. 



III. The only deep-water currents of sufficient strength 

 to scour away or prevent deposits are those about sub- 

 marine ridges impeding the circulation of polar and 

 equatorial waters. 



IV. The material of shore deposits has a tendency to 

 travel to and fro along the coast with waves and currents, 

 the maximum action of which is between low and high- 

 water mark, and every hollow in the sea-bed, near the 

 shore, acts as silt-trap for travelling sediments. 



Applying these principles to the case under review, 

 assummg, in the absence of evidence to the contrary, that 

 every part of the small Adour basin was subjected to the 

 same total amount of subsidence, which, measured at the 

 old river mouth at the foot of the continental escarpment, 

 appears to be about nine thousand feet, and dividing the 

 period of subsidence into two : — 



(a.) Period before submergence of the edge of the conti- 

 nental platform — here, apparently, indicated by the two 

 hundred fathom line — and with it of the terminal head- 

 lands of the Adour valley. 



(r;.l Period after it. 



It follows that, at the beginning of period (a), the sea- 

 shore was at the base level of erosion or the foot of the 

 continental escarpment, and the whole region drained by 

 the Adour was some nine thousand feet higher above sea- 

 level than it now is. It was therefore, better able to inter- 

 cept moisture-laden air-currents. Precipitation was accord- 

 ingly, heavier, and the drainage area including, as it did, a 

 region now submerged, was considerably larger than now. 

 The stream's volume must, therefore, have been much 

 greater than at the present time. 



The submerged part of the channel presents the very 

 steep fall of from sixty to seventy feet per mile, so the 

 Adour was, then, more a torrent than a river. Its erosive 

 power must have been very great and the amount of sedi- 

 ment borne along very large. The narrowness of the 

 submerged valley — a mere gorge — shows that the rate of 

 erosion of the bed was much more rapid than that of 

 weathering of the sides. 



As subsidence progressed the gorge must have assumed 

 the aspect of a long fiord, which, under conditions of slow 

 subsidence, would have soon been filled up with the river's 

 own deposits to the level at which scouring action of tidal 

 currents obtained the mastery — that is, up to, say, the 

 fifty-fathom line. As nothing of the kind seems to have 

 happened, it must be inferred that either the scouring 

 action of the tidal currents extended, here, to a depth far 

 greater than it is known to do anywhere else on the earth, 

 or the rapidity of submergence was such that the amount 

 of sediment brought down by the river during the process 

 was insignificant. 



Further, it has been observed that the depth of the sea 

 within a fiord often far exceeds that just without it — this 

 being due to the filling in of the trough, up to scour limit, 

 by the sediments ever traveUing up and down the coast. 

 As the process does not extend far into the fiord, the result 

 is a kind of deep bar at the mouth. As there is apparently 

 no such bar in the case of the submerged mouth of the 

 Adcur valley, it must be inferred that either there was no 

 shore deposit going on at the time, or the rapidity of 

 submergence was such that the amount of travelling coast 

 sediment trapped by the deep trough during the process 

 was insignificant. 



At the close of period (a), the Adour fiord had, at the 



