231 



claim from us continued exertions to investigate both the physical condition 

 of the rocks and history of the species; and again, I know of no series of 

 beds which determine to us the secular movements, or gradual oscillations 

 of land, and relatively the water, than these transitional accumulations 

 during late or post-Triassic or pre-Liassic age. 



The group of rocks, and section at Westbury, help us by theii" 

 geographical position, as well as geological hoi-izon, to examine and 

 understand the conditions of the Keuper Marls at their close, and the 

 succeecUng and alUed Rhsetic series upon them, and also the base of 

 the Lower Lias, as evidenced by the Planorbis Zone at the summit of the 

 cliff and seen in position, at its junction with the Transitional or Ehsetic 

 Zone below." 



It is also one of the few places where evidence is clearly afforded 

 which may guide us to a right understanding of the many fluctuations and 

 i-ecurring uniformities, arising from slow oscillation of the land, 

 and deposition of organic and inorganic matter, and the coiTesponding 

 development of life in an estuarine, or shallow sea. For I cannot 

 conceive that the Rhstic series could have accumulated, or have 

 been deposited imder any other conditions, than on a slowly-descending sea 

 bed, and repeated pauses (as evidenced by the numerous Bone beds and 

 crystalline lime bands), extending over great intervals of time, which thus 

 gradually and steadily produced the alternating conditions so manifest in 

 the physical structure of the shales, marls, and thin limestones of the 

 beds comprising the structure of this section in particular and others 



generally. 



DISTRIBUTION. 



This thin, and to some apparently insignificant section on the banks 

 of the Severn, at Westbury, as well as two or three others which occur 

 in the magnificent valley watered by this grand river, are English types 

 of the extensively developed rocks of the same age and physical aspect 

 in Austria, Lombardy, France, Hanover, Savoy, Saxony, Bavaria, and 

 Switzerland, &c. ; and although with us the series is never found to exceed 

 one hundred feet in thickness, including (according to our new reading) 

 the grey, green, and white marls of the Upper Trias, yet the fact of 

 its liighly developed Palaeontological features agreeing in almost every 

 essential character with the Continental deposits, enables us to co-ordinate 

 through its Fauna, the entire series in this countiy with those of Europe 



* The only complete series exliibitiug all these groups in succession is seen near 

 Watchett, where every bed, from the Tea green Marls of the Upper Trias, to the 

 Bucklandi Zone, is measurable in one cliff section. 



