﻿83 MR. S. S. DUCKMAN OX CERTAIN [Feb. I9IO, 



Table VI. — Migration of Areas of Development in the Toarcian. 



Approximate 

 Zones. Localities. development. 



Feet. 



d fy° orel ■:•■■:-) South Dorset 200 



E 1 d j s P™ m6m :•• 1 Mid and North Somerset 100 x 



g I sfruckmanni . | 



*^ \__striaUiliiS . 



r variabilis ...\ South Cotteswolds 240 



uiu 



J 



5 <^ subcdrinatum. J 



braunianum . 1 



fibidatuin ;- Northamptonshire 150 2 



id 



ovatnm 



iahifcnt hi 



exaraiuin j 



vtenuicostatum. J 



Yorkshire 160 



Total 850 



Owing to this migration of area of maximum deposit, it happens 

 that the strata of the Toarcian in an}' one English locality do not 

 exceed much over 250 feet in thickness, and are often far less ; yet 

 the amount of work done in deposition during that time is equal 

 to 850 feet or more. 



Though the Toarcian is now divided into fourteen zones, these 

 zones can hardly be called minute divisions, when some of them 

 develop thicknesses of 100 or more feet each and maintain these for 

 many miles. 



Since the Toarcian thus contains so many zones, it is often 

 necessary to speak of the earlier of these zones as distinct from 

 the later, or vice versa, and as the circumlocutory phrases pre- 

 striatulus Toarcian, striatulus and \)ost-striatulus Toarcian are very 

 awkward, it is now suggested that the former be called Whitbian 

 and the latter Yeovilian. The Whitbian would contain the zones 

 tenuicostatum to variabilis inclusive, all of which are more or less 

 finely developed on the Yorkshire coast in the neighbourhood of 

 Whitby ; while the Yeovilian would contain the zones striatulum 

 to moorei inclusive, and certain of these are remarkably developed 

 in the Somerset-Dorset district. 



The special faunal feature of the Whitbian is the development of 

 the Lias planulati (Dactylioidae, Hyatt) which are very numerous 

 in most of the zones ; while the feature of the Yeovilian is the 

 absence of all Dactylioida?, but the development of Grammoceratinae, 

 of Hammatoceratida?, and of Dumortieria. 



The names would mark another distinction — the difference 

 between the Cotteswold and other sands in date. Thus, the Cot- 

 teswold Sands being pre-striatulan would be Whitbian ; but the 



1 Estimate, data uncertain. 



2 Average thickness according to Mr. Beeby Thompson. 



