208 



LYELL'S ELEMENTS OF GEOLOGY. 



Wealden Strat a formed in Shallow Water. 



the sea in which the corals and shells of the oolite grew ; then, 

 land, which supported a vegetable soil with Cycadese ; then, a 

 lake or estuary, in which freshwater strata were deposited ; then, 

 again, land, on which other Cycadese and a forest of dicotyledo- 

 nous trees flourished ; then, a second submergence under fresh- 

 water, in which the wealden strata were gradually formed ; and, 

 finally, in the cretaceous period, a return over the same space 

 of the ocean. 



To imagine such a series of events will appear extravagant 

 and visionary to some who are not aware that similar changes 

 occur in the ordinary course of nature ; and that large areas 

 near the sea are now subject to be laid dry, and then submerged, 

 after remaining years covered with houses and trees.* 



In some of these modern revolutions, such as have been wit- 

 nessed in the delta of the Indus, in Cutch, we have instances of 

 land being permanently laid under the waters, both of the river 

 and the sea, without the soil and its shrubs being swept away ; 

 but such preservation of an ancient soil must be a rare exception 

 to the general rule, for it would be destroyed by denuding waves 

 and currents, unless the land sank suddenly down to a great 

 depth, or unless its form was such as to exclude the free ingress 

 of the sea. 



Notwithstanding the enormous thickness of the wealden, ex- 

 ceeding in some places perhaps 1000 feet, there are many 

 grounds for believing that the whole of it was a deposit in water 

 of moderate depth, and often extremely shallow. This idea may- 

 seem startling at first, yet such would be the natural consequence 

 of a gradual and continuous sinking of the ground in an estuary 

 or bay, into which a great river discharged its turbid waters. 

 By each foot of subsidence, the fundamental rock, such as the 

 Portland oolite, would be depressed one foot farther from the sur- 

 face of the ocean ; but the bay would not be deepened if new 

 strata of mud and sand should raise the bottom one foot. On 

 the contrary, such sand and mud might be frequently laid dry 

 at low water, or overgrown for a season by a vegetation proper 

 to marshes. At different heights in the Hastings Sand in the 

 middle of the Wealden, we find again and again slabs of sand- 

 stone with a strong ripple-mark, and between these slabs beds 

 of clay many yards thick. In some places, as at Stammerham, 

 near Horsham, there are indications of this clay having been 

 exposed so as to dry and crack before the next layer was thrown 

 down upon it. The open cracks in the clay have served as 



* For an account of recent movements of land attended by such conse- 

 quences, see Principles of Geology, Index, " Cutch," " Sindree," &c. 



