TBAXSACTIONS OF SECTION C. 541 



taneously a subsidence of the earth's crust; though hut little eiJbrt has been made 

 to determine whether they are dependent on each other as cause and effect. 



Borings in deltas prove that depression to a great extent has occurred whilst 

 the accumulation was being deposited. The greater amount of detritus derived 

 from hills and valleys is canied into the sea, but. instead of filling it up, the water 

 becomes of a great depth at a few miles from the mouths of large rivers. 



There was a progressive subsidence of the land during the Glacial period ; this 

 may be ascribed to the weight of accumulated snow, and of the newly-formed 

 boulder-clay ; a similar depression is occurring in Greenland, under a rapid increase 

 of snow. 



The carboniferous series above the limestone afford most satisfactory evidence 

 that the amount of subsidence coincides with tliat of deposition ; the surface of the 

 limestone and the beds of coal furnishing sufficiently correct base-lines for deter- 

 mining the question. Near its southern boundary in Coalbrookdale, the carboni- 

 ferous limestone is overlaid by millstone grit, the thickness of which, as well as of 

 the coal-measures, is there very inconsiderable, compared with localities in Den- 

 bighshire and Lancashire. There is also apparent in numerous instances, even 

 within a distance of a few miles, a considerable difference in the thickness of the 

 strata between identical beds of coal. The most remarkable example recorded is 

 that of the 'Thick,' or 'Ten-yard' coal of Dudley, which, from thirty feet of 

 coal, and with two to four feet of ' partings,' has five miles towards the north 

 separated into ten or twelve beds of coal, which combined are of the same thick- 

 ness, but with the intermediate measures amount to 406 feet. — {Jukes.) 



There must needs be a cause for this universal occurrence of subsidence with 

 deposition of strata, the only ethcient one being the weight of the accumulated 

 material pressing down the crust of the earth resting upon a iluid substratum. 



Elevation also happens on the removal of pressure, and 'those regions which 

 have suffered the greatest amount of denudation have been elevated most-' — 

 {Captain Button, U.S. Ordnance Survey.) 



At the termination of the Glacial period, the land, depressed b)' its load of 

 snow, became, upon this melting away, re-elevated to a certain extent. This, and 

 the rising of the land at the preseut time in Norway and Spitzbergen, may be 

 attributed to the removal of a thick covering of snow. 



In elevated districts the highest parts are those in which there has been the 

 greatest amoimt of denudation, and often consist of the lowest rocks in a geological 

 series. Thus in North Derbyshire the highest laud is composed of carboniferous 

 limestone, from above which more than o,0(J0 feet of later carboniferous strata have 

 been removed. 



The author thinks that these depressions and elevations cannot be ascribed to 

 secular cooling of the mass of the earth, since by such action the accumulation 

 cannot also be accounted for ; nor could the same agency acting only in one direction 

 cause both depression and upheaval. 



The concurrent phenomena of accumulation and subsidence, and their converse, 

 demand serious and careful investigation ; especially as in them may be found 

 the great moving-power upon which depends the greater number of geological 

 changes. 



8. On the Cause of Elevation and Siihsidence of Land. 

 By J. S. Gardner, F.G.S. 



The paper c'aims that tlie evidence of the permanence of continents is iacon- 

 clusive as regards eocene and prre-eocene periods, and inquires what the shallower 

 regions of the Atlantic mean, if they do not mean a change of level at tlie .sea- 

 bottom. Assuming, with Sir C. Lyell, that at a given depth rocks are molten, and 

 that under further pressure they are reconverted into solids of high specitic gravity, 

 the paper demonstrates that the outer envelojie is susceptible to and gives way under 

 any increased weight, and recovers when tliis is removed. The evidence relied upon 



