3 2 4 



NATURE 



\August 2, 1883 



rippled and sun-cracked surfaces, marked often with 

 burrows and trails of worms, as well as the prevalent 

 character of their organic remains, show that they must 

 have been deposited in areas of slow subsidence, bordering 

 continental or insular masses of land. 1 Vast thicknesses of 

 strata have been continuously deposited at or near the sea- 

 level. The coal-measures present a series of alternating 

 layers of vegetable matter and brackish water sediments, 

 reaching in the South Wales district a thickness of upwards 

 of 10,000 feet, whose accumulation must have been accom- 

 panied almost foot by foot by a corresponding subsidence. 

 The Cambrian sediments accumulated in the British area 

 to a thickness of 23,000 feet apparently without any 

 gieat change in the depth of the sea in which they were 

 formed ; and throughout the deposition of the whole of 

 the Silurians, subsidence seems to have kept pace with 

 sedimentation. The Permian again furnishes many in- 

 stances of sedimentation at or near the sea level sustained 

 throughout great thicknesses, and of frequent alterna- 

 tion of marine and freshwater deposits. Among Meso- 

 zoic rocks, the New Red Sandstone furnishes an example 

 of isolated basins of deposit to which the sea found 

 repeated access, though a thickness in places of 3000 feet 

 had accumulated in them. The German Triassic basin 

 is for the greater part of its thickness a succession of ter- 

 restrial deposits containing plant-remains. The Jurassic 

 and Cretaceous systems were deposited during inter- 

 rupted depression of the sea bottom, while the Tertiaries 

 abound with local instances in which subsidence has kept 

 pace with sedimentation. 



The washing of sea-coasts and removal of material 

 shown by the discoloration of the sea for miles round the 

 shore in stormy weather, shows that the process of accu- 

 mulation of sediment still progresses on a very large 

 scale.' 2 It has been ascertained that nearly the whole of 

 this must be rcdeposited within a distance of thirty miles. 

 If the waves have no disturbing power at a greater 

 depth than 40 feet, and could therefore neither deepen 

 the sea-bed nor prevent its silting up to within that 

 depth, our shores should be surrounded by enormous 

 tracts of shoal water, whose bottom might be grooved or 

 deepened by local currents, but whose average depth 

 would not exceed 40 feet, or even less, since many rocks 

 are so protected by seaweed that their further degrada- 

 tion when once below the reach of surf must be inappre- 

 ciable. There is no cause therefore capable of generally 

 deepening the sea round coasts beyond some such limit 

 as this, except subsidence, and this can only be ascribed 

 with any semblance of probability to the accumulating 

 weight of sediment. The prevailing tendency on sea- 

 margins is and must be towards depression, and there 

 are few residents on the sea-coast who would be unable 

 to contribute valuable observations on this point. It 

 must however be remembered that wh,le raised beaches 

 are conspicuous objects, depressed beaches could ob- 

 viously hardly ever attract attention, even if the shingle 

 had not been removed by the surf, and further, most, if 

 not all, of the existing raised beaches may have been 

 formed during the general elevation of the land that took 

 place at the close of the glacial period. 3 Other observers 



I " Text- Book of Geohgy," Geikie. 1882, p 647. 

 M. Marchal has estimated that the sea deposits annually 6jo,c 

 metrescf sediments in the Kay of Mm Si. Michel, and 10,000,000 on the 

 coasts of Flanders, Zealand, and Norfolk. 



3 My own experience on the south coa^t i- that the weight "f evidence 

 points to a general sinking, for vestiges of submerged land vegetation and 

 traditions of submergence are very frequent. At Bournemouth I have seen 

 heath plants and roots, fresh-looking except for the incipient formation of 

 pyrites, cast up from a tract of moorland now below the sea-level. Poole 

 Harbour would lung since have been left by the sea if there were no subsid- 

 ence, and a landing-stage with rings found below low-water mark furnish 

 valuable data as to the arm lint that has taken place in historic times The 

 Solent must have been urig.nally a harbour like that of Poole, continually 

 silting, s.nking, and enlarging, the depression travelling west and cutting one 

 nverafter another from the sea until the western channel was at last opened. 

 The immense accumulations of mud in its channel seem to have dragged the 

 land into a sort of trough with raised sides, s, that the Yar, Medina, and 

 Brading Rivers flow inland ii stead of out to sea. On its margins we find 

 here have been oscillations of level, caused perhaps by alterations in the 



have recorded numerous submerged forests on the coasts 

 of Cornwall, Devon, Somersetshire, and Wales. An ele- 

 vation of the coast may, on the other hand, be sometimes 

 accounted for when due consideration is given to the sur- 

 rounding conditions. 1 It may, for instance, conceivably 

 be produced on any shores where considerable sediments 

 are forming at some distance out to sea or where masses 

 of cliff are being washed away. 



The extreme sensitiveness of the earth's crust to any 

 changes in the distribution of weight on its surface is, 

 however, best exemplified by those local depositions and 

 removals of matter which have attracted more general 

 attention at the present day. The chief of these is the 

 transfer of matter by river action from large tracts, and its 

 accumulation in such limited areas as plains, estuaries, 

 and deltas. Borings of 400 and 500 feet have shown that 

 these often consist of long successions of silts, which 

 alternating layers of shells and of vegetable matter prove 

 to have been deposited at or near the sea-level, and the 

 Wealden and Eocene formations in the British area show 

 that such accumulations may exceed 1000 feet in thick- 

 ness. In the case of deltas, subsidence must keep pace 

 almost foot by foot with the accumulation, and be con- 

 fined to the area over which the sediment is being de- 

 posited, for anymore rapid subsidence would check its 

 growth and convert it into an estuary. This sinking is . 

 apparently of universal occurrence. 



A similar instance of the transfer of weight from larger 

 areas and its precipitation on a very circumscribed area 2 

 is seen in coral atolls and reefs. The explanation of their 

 formation given by Darwin requires a gradual subsidence 

 keeping pace with their growth, which takes place within 

 twenty fathoms of the surface only. This theory, simple 

 and admirable as it is, accounting satisfactorily for all the 

 observed phenomena of coral growth, has been contested 

 by Mr. Murray, who has shown that atolls might be 

 merely incrustations of volcanic peaks. But his theory 

 seems improbable by contrast, for it demands 290 vol- 

 canic peaks at the sea-level in the Pacific coral area alone, 

 every foot of which has been completely concealed by 

 coral growth, though few volcanic craters are known so 

 near the sea-level outside this area. We seem thus to 

 have in coral growths another evidence of subsidence 

 keeping pace with the increase of weight, sometimes, as 

 soundings prove, to a depth of 1000 feet or more. The 

 replacement of a column of sea water 100 fathoms in 

 depth, by a column of limestone, would increase the pres- 

 sure per square fathom from 6io,i tons to 1487 tons, so 

 that it is easy to realise how vast must be the increased 

 pressure on such an area as that occupied by the great 

 reef of Australia, 1250 miles long and 10 to 90 miles broad. 



The sands, gravels, and clays, with marine shells and 

 erratic boulders, prove that a great submergence took 

 place during the Glacial Period, while Europe was under 

 an ice sheet 6000 feet thick in Norway, and diminishing 

 to 1500 in Central Germany. The extent of the submer- 

 gence has been perhaps understated at 600 feet in Scan- 

 dinavia, and was at least 1350 feet in Wales. A corre- 

 sponding re-elevation accompanied the disappearance of 

 the ice. It has often been supposed that the sinking of 

 the west coast of Greenland is similarly due to its ice-cap. 



It is probable that great outflows of lava may in like 

 manner occasion subsidence — though it is by no means 



position of the sediments through changing currents. The inroads of the sea 

 at Paeham and Selseashow the downward movement to have extended along 

 the whole of the Hampshire coast. - 



1 An elevation, for instance, has taken place on the Kentish coast which 

 has closed the Slour to navigation and caused the sea to retreat from Stour- 

 mouth. RicnboTOUgb, and Sandwich, and which is als:> marked by the great 

 v p. isures of Eocene along this part of the coast above low- water mark, and 

 n!d hardly exist where exposed to strong tidal and wave action, un- 

 less the abrading process were counteracted. The immense deposits, taking 

 place at a distance from shore, brought down by the Thames, must lead to 

 con [derable subsidence in its estuary and consequently some corresponding 

 elevation along its shores. The Thames sediment is of unknown depth, hut 

 on its iiiarg.ns at Sheerness the alluvia! mud is 80 feet thick, and at Up- 

 church, opposite Queenborough, 75 feet. 



- But 3& per cent, of sclids preexisted in the water displaced by the rock. 



