Ch. XX.] PUEBECK BEDS. 393 



or from these again to a state of land, which have occurred in this 

 part of England, between the Oolitic and Cretaceous periods. That 

 there have been at least four changes in the species of testacea during 

 the deposition of the "Wealden and Purbeck beds, seems to follow from 

 the observations recently made by Professor Forbes ; so that, should 

 we hereafter find the signs of many more alternate occupations of the 

 same area by different elements, it is no more than we might expect. 

 Even during a small part of a zoological period, not sufficient to allow 

 time for many species to die out, we find that the same area has been 

 laid dry, and then submerged, and then again laid dry, as in the 

 Deltas of the Po and Ganges, the history of which has been brought 

 to light by Artesian borings.'" We also know that similar revolutions 

 have occurred within the present century (1819) in the delta of the 

 Indus iu Cutch,f where land has been laid permanently under the 

 waters both of the river and sea, without its soil or shrubs having been 

 swept away. Even independently of any vertical movements of the 

 ground, we see in the principal deltas, such as that of the Mississippi, 

 that the sea extends its salt waters annually for many months over 

 considerable spaces which, at other seasons, are occupied by the river 

 during its inundations. 



It will be observed that the division of the Purbecks into upper, 

 middle, and lower has been made by Professor Forbes strictly on the 

 principle of the entire distinctness of the species of organic remains 

 which they include. The lines of demarcation are not lines of dis- 

 turbance, nor indicated by any striking physical characters or mineral 

 changes. The features which attract the eye in the Purbecks, such 

 as the dirt-beds, the dislocated strata at Lulworth, and the Cinder- 

 bed, do not indicate any breaks in the distribution of organized beings. 

 " The causes which led to a complete change of life three times during 

 the deposition of the freshwater and brackish strata must," says this 

 naturalist, " be sought for, not simply in either a rapid or a sudden 

 change of their area into land or sea, but in the great lapse of time 

 which intervened between the epochs of deposition at certain periods 

 during their formation." 



Each dirt-bed may, no doubt, be the memorial of many thousand 

 years or centuries, because we find that 2 or 3 feet of vegetable soil 

 is the only monument which many a tropical forest has left of its 

 existence ever since the ground on which it now stands was first cov- 

 ered with its shade. Yet, even if we imagine the fossil soils of the 

 Lower Purbeck to represent as many ages, we need not expect on that 

 account to find them constituting the lines of separation between suc- 

 cessive strata characterized by different zoological types. The pres- 

 ervation of a layer of vegetable soil, when in the act of being sub- 

 merged, must be regarded as a rare exception to a general rule. It is 

 of so perishable a nature, that it must usually be earned away by the 



* See Principles of Geol, 9th ed., pp. 255, 275. f Ibid., p. 460. 



