Jan. 6, 1876] 



NATURE 



199 



Cambridge valley, A short description of the geology and 



Ehysical features of the district was given, and an inference dra'n-n 

 om the varjing faunas of the " Crag" that the land was sink- 

 ing durirg that era and until after the deposition of the Cromer 

 "Forest-bed." Then Arctic conditions began to prevail, and 

 the great glacial subsidence commenced; the "lower glacial" 

 beds were foinred, and succeeded by a large series of false-bedded 

 grave's atd sands, with intercalated patches of unstratified clay. 

 These deposits run up only to a certain level, about 300 feet, 

 Eever quite reaching the top of the chalk escarpment, where 

 the overlying boulder-clay is invariably found resting on the 

 older rock, without any gravel or sand between. The author 

 inferred from this circumstance that after the deposition of the 

 ' ' lower ■' beds, and as submergence went on, the waters of the 

 North Sea were again united to those of the Atlantic A strong 

 current was thus set up, which swept down from the north, 

 bringing with it the material of which the gravels are composed, 

 and which is found to consist of pebbles, all derived from the 

 northern and eastern coasts, mixed with flints from the chalk. 

 The escarpment of this formation stood at the time above the 

 water, but when once sufficiently submerged to admit the water 

 over its lower portions, the conditions were altered, the current 

 lost its force, and the deposition of gravel ceased. An oc- 

 casional iceberg had dropped its load of un-tratified clay, which 

 became intercalated with the gravels, but the greater number of 

 such bergs were quickly swept away to the south. Now the 

 waters had access to a larger area, the formation of gravel was 

 succeeded by that of boulder-clay, which in the author's opinion 

 is entirely composed of masses of clay enclosing boulders, brought 

 down ai' J dropped by icebergs in mass, which accounts for its 

 want 0. stratification. This boulder-clay rests evenly on and 

 at the h .her'level overlaps the "middle glacial" sands ; it then 

 caps the chalk escarpment and plunges down into the Cambridge 

 vailey, even to the present level of the sea ; but in no instance 

 on or be) ond the escarpment does any sand or gravel intervene 

 between it and the older geological formations, although just 

 over itie scarp (on the south side) the gravels run up to an 

 elevation of 300 feet. The gravel- forming currents were 

 evidently cocimed to the seaward side of the chalk range, and 

 excluded from the Cambridge valley, which is undoubtedly pre- 

 gladal, and which formed at the time a large inlet, land-locked 

 on every side but one, discharging its waters through the open- 

 ing now occupied by the estuary of "The Wash." In the 

 Cambridge vaUey there are sheets of river-gravel of recent date, 

 some patches of doubtful age, but not traceable under the 

 boulder-clay, and an elongated series of gravels at a level of 20 

 to 60 feet above the present level of the Cam. These are in some 

 parts distant from the present course of the river, and present a 

 striking resemblance to glacial gravels ; but as they here and 

 there contain recent shells, and taking into consideration their 

 uniformity of level, the author concludes that they indicate an 

 ancient course of the River Cam. The other conclusions 

 arrived at, after mature consideration of all the e^ndence hitherto 

 obtained, are — that a gradual passage will be fotmd to exist 

 fi-niTi the base of the crag up to and through the drift-deposits 

 ihose of recent date ; that in East AngUa we have evidence 

 but one, and that a gradual period of glacial submergence 

 ceeded by a corresponding movement of re-elevation; and 

 t there are no " middle glacial " deposits whatever within the 

 a of the Cambiidge valley. — " Denuding Agencies and Geo- 

 ical Deposition under the Flow of Ice and Water, with the 

 .vs which regulate these actions, and the special bearing on 

 .r-action, of observations on the Mississippi and other great 

 jrs, and their present and past Meteorological conditions, and 

 ilar remarks on Marine Deposits, illustrated by the Irish 

 a and the Chesil Beach," by Mr. A. Tylor. The writer ad- 

 duced evidence by measured sections and drawings to show that 

 the Quaternary gravels were deposited rather in a wet or pluvial 

 than in a snowy or Glacial period. He Ihought the denuding 

 action of springs and the alternate action of rain and frost had 

 been neglected. He considered Agassiz and other writers had 

 overlooked the previous writings of Playfair, to whom he re- 

 ; fened. The rainfall of Westmoreland, Switzerland, and the 

 I Mississippi valley were compared in summer and winter to prove 

 that floods were not necessarily greater from land covered with 

 snow than from land covered with trees and vegetation when 

 1 height above the sea and local circumstances weie taken into 

 consideration. Mr. Dana's "Great Glacier," whose melting 

 ^s to supply a Quaternary liver, Mississippi, 50 miles wide, 

 i^uld require a supply equal to 625 times the present rainfall to 

 till it. The melting of snow was assumed to b€ of such pro- 



portions by modem writers as to e<:itial the debacles of older 

 geologists. The high Swiss mountains pointed to a greater 

 diminution of snow on high groxmd in the Glacial period ; and 

 he believed the clouds then discharged near the sea-level, so that 

 the mass of snow and ice was at low levels. It appears that in 

 Greenland in the 80th parallel, according to Nordenskiold, near 

 the sea in summer there is no sn on the ground i,cxx) or 1,500 

 feet above the sea. Open water at the poles must depend upon the 

 abstraction of the vapour from the atmosphere at lower lati- 

 tudes ; and probably in the Glacial period the ice-cap was thickest 

 at the 70th parallel of latitude, Mr. Tylor thought the theories 

 of former depressions of the land, as in the Mississippi valley, 

 should be tested by examination for flexures. He had found 

 (in 1868) that flexures, and fracttires, had very much affected the 

 course of the Wealden denudation in the Quaternary period. 

 The laws of river motion are very simple and precise ; and as de- 

 pressions and upheavals are always imequal, any great move- 

 ments in the Quaternary period would afliect the courses of rivers, 

 and be traceable in their deposits. The author had measured the 

 remanies valley gravels of Coalbrook Dale, which were associated 

 with marine shells 200 feet above the sea, and compared their 

 contour with ordinary valley gravels and with marine beaches, 

 to ascertain under what probable conditions the sea had risen up 

 the Severn valley without leaving any traces of clifis or marine 

 denudation except between Bridgnorth and Coalbrook Dale. 

 The diamond gravel-deposits in Africa have a similar contour 

 to those of Coalbrook Dale. The position of the Moel Tryfaen 

 beds was first described by Trimmer in 1831. Trimmer, an ex- 

 cellent geologist, observed the scratches on the rocks covered 

 by the gravel with marine remains, and noticed thtir ice-')rig n, 

 but did not draw, unfortimately, the natural inference that f ere 

 must have been a Glacial period in Wales. This great d scovcry 

 or invention was left to Agassiz to propose in 1837. The 

 glacier-eroded lakes, much lower than Moel Tryfaen, and close 

 to it, are free from marine remains, therefore it seems difiicult 

 to suppose a depression of 1,300 feet and immersion in the sea 

 of Tryfaen, and subsequent elevation, could have taken place 

 without having left any marks on the land except at one spot. 

 The measured section of the Chesil Beach shows its close ap- 

 proximation to a binomial curve, and the regularity of beaches 

 and littoral zones along the Channel teach us what are the 

 certain consequences of land being immersed tmder the sea. 

 Mr, Tylor produced plans and sections showing how the tide 

 actually affects the sea-bottom, and described the gorge below 50 

 fathoms in the Irish Sea. He treated the tide as caused by the 

 alternate and opposite slow movement of the deep and great mass 

 of the Atlantic, giving motion to the water at the coast almost 

 simultaneously as if the whole water moved as one mass over an 

 area of thousands of square miles. The velocity of the tide of 

 one tenth of a mile per hour in a deep sea, produced by the com- 

 position of forces a tide of a velocity of three or four miles an 

 hour on the coast. High and low water at difl^erent ports are the 

 direct consequences of local cxxrrents in shallow water, set in 

 motion by the greater mass of deep water. There are points 

 in the English Channel where within a few mUes there is a 

 difference of six hours in high water. He objected to the theory 

 of a tidal wave travelling in one direction, and moving faster in 

 deep water than in shallow, because the tide really travels quicker 

 in shallow water, as his plans show. In support of this he 

 showed the chart of the Channel, and that the tide turned in the 

 Irish Sea at all points, deep or shallow, almost simultaneously 

 and synchronously with the slow tidal movement in the Atlantic. 

 He found that in a large area of sea of 120,000 square miles, 

 where the water averaged 67 fathoms off the Sciily Islands, the 

 velocity of the tide was only one mile per hour, but in the 

 shallows near the Chaimel Islands, where the depth was on an 

 average 12 fathoms, by the composition of forces the velocity of 

 the tide increased to 6 miles an hour. If the tide was the con- 

 sequence of a tidal wave bringing high water, the tidal conditions 

 of the Irish Sea would be very different from what they are de- 

 scribed to be. He did not find any evidence of a plane of 

 denudation on any sea coast, but, on the contrary, deep gorges 

 and curved surfaces, depth varying with width, &c. The nearest 

 approach to a plane surface was in the estuary of the La Plata ; 

 but that flatness appeared more the consequence of deposition 

 than denudation. The great cuts or indentations out of coast 

 lines where rivers discharge into the ocean, when compared 

 with the absence of indentations in areas where there are no 

 great rivers, but where the rocks are equally hard, showed that 

 such denudation depended upon the alternate and opposite action 

 of rivers and the tide. He referred to the removal of the bar 



