48 GEOLOGY OF THE ISLK OF WIGHT. 



is the habit of animals of such genera as Pinna and PunoptBa to 

 assume when alive.* 



" After this a temporary change followed, when an influx of 

 sand, mingling with the calcareous mud, caused a state of sea- 

 bottom peculiarly favourable to the presence of animal life. In 

 this way were called into existence a multitude of species which 

 were added to those which had appeared before them. This was, 

 in fact, such a state of sea-bottom as is novv' presented by great 

 shell-banks ; but it does not seem to have lasted loner, and new 

 depositions of mud appear to have extinguished some forms, whilst 

 others suffered by the change only in the diminution of their 

 numbers. In the midst of this muddy epoch, a temporary and 

 peculiar condition of sea-bottom, forming what are now called the 

 Crackers, called forth the presence of numerous mollusca, at first 

 of various species of the genus Gervillia, and afterwards of 

 Auricula \^Avellana~\, Cerithium, Dentalium, and other univalves, 

 which appear to iiave enjoyed but a brief existence(as species)in this 

 locality, since similar conditions were never afterwards repeated 

 The greater number of the Gasteropodous mollusca of the English 

 Lower Greensand are found within this very limited range. At 

 the close of the deposition of this great mass of clay there was for 

 a time a great multiplication of the individuals of certain Brachio- 

 poda, which had commenced their existence in the lowest beds. 

 Thus Terehratula Gibbsii \Iihynchonella (jihhsiand\ suddenly 

 appears in immense abundance, covering the bottom of the sea, 

 and predominating over the animals among which it had previously 

 been but thinly scattered. 



*' This lowest zone of TerehratulcB marks the commencement of 

 a new state of sea-bottom where sands predominated over the 

 clays, each interval of deposition being usually marked by the 

 presence of a layer of GryphcBa \_Exogyra'\ sinuata, the period of 

 rest being almost always sufficient to enable the Gryphaa to 

 attain its full growth. Other bivalves are found with it, but in 

 comparatively small numbers, and not such as are of gregariuus 

 habits. During the whole of this period enormous Gejilialopoda, 

 includinti; species of Crioceras and Scaphites [Ancyloccrasi, fre- 

 quented these seas, and when dead formed the nuclei round which 

 calcareous and sandy matter collected and formed nodules. The 

 death of these animals seems to have been connected with the 

 periodical charging of the sea with sediment ; hence we find them 

 usually alternating with the zones of Gryplicea, and forming 

 irregular bands in the intervening sedimentary deposits. 



* •' The saTDC decided chauge from dark-coloured fresh water marls containing 

 Melanopsis (or Melania) [ Vicariia~\ and Cypris to marine beds, occurs round the 

 edge of the Weald, and was very well exposed at Ilaslemere during the cutting of 

 the London and Portsniouth Kaihvay, a lew years back. In company with Professor 

 llamsay and Mr. F. Drew, I examined the passage beds, and found in the brown 

 clay abundant tracks of marine worms, and the Pdnopaa, vertical in their old 

 burrows, within an inch or two of the dark marls. A great Pcnia, a coral {Ho/o- 

 ci/slts elegans), and numerous other fossils, occur in plenty just above these." — 

 J. W. Saltkk. See Geology of the Weald, p. 114 (Mem. Geol. Survci/). 



