part 4] JURASSIC cheonologx. 427 



It is fairly certain that, when the fauna of the strata now dated 

 as niortense are submitted to analysis, several episodes will be 

 found covered by the one name — deposits of one episode have 

 been preserved at one place, and those of others in other places. 

 And yet the sequence may not be complete. 



These remarks are made to show that, on theoretical grounds, 

 there is reason to claim plenty of time between the dates of the 

 deposition of the lied Bed and the Astarte Bed for the various 

 events of elevation, denudation, deposition, to have occurred even 

 in repetition. And there is actual evidence in support : in the 

 Hebrides, according to the researches of Dr. Gr. W. Lee, some 

 70 feet of strata separate deposits which are respectively the 

 equivalents of the Ked Bed and the Astarte Bed. Unfortu- 

 natel}", they are unfossiliferous, and so their dating as niortense 

 rests only on the fact that they are between hlagdeni and 

 Garantiana hemerse. Then in the Sherborne district there are 

 poorly fossiliferous strata — the Building-Stones — which are post- 

 niortense : they are reckoned as belonging to the Garantiana 

 hemera, and so is the Astarte Bed of Burton Bradstock ; but that 

 they are strictly contemporaneous is possibly doubtful — the various 

 Garantiana-Yike forms in the two deposits show certain differences, 

 and therefore, at any rate, the species are not altogether identical. 

 Possibly the Astarte Bed is, in part at least, of somewhat earlier 

 date than the Building- Stone of Sherborne ; but it is unadvisable 

 to speak positivel}^ on this point, until the Garantiana -like forms 

 can be worked out in detail — a long task with such a mass of 

 material as the strata yield. 



The point to be emphasized is the necessity of knowing whether 

 our chronological datum-lines are strictly contemporaneous ; because, 

 as it is onl}^ possible to estimate time by the amount of work per- 

 formed during stated periods, it is necessary to be certain that the 

 periods are identical. For instance, was the thickness of 70 feet 

 in the Hebrides laid down only prior to the Building-Stone of 

 Sherborne, or was it laid down prior to the Astarte Bed of Burton, 

 or, as is possible, was it laid down even prior to the White Bed of 

 Burton ? These possible differences make considerable difference 

 to the time-estimate ; for, in the last case, the 70 feet represent 

 work done in a time-interval between the Bed Bed and the White 

 Bed, which w^as a period of upheaval and erosion at Burton ; then 

 between the Bed Bed and the Astarte Bed, which reposes upon it, 

 one could place a time-interval represented by work done in 

 depositing 70 feet in the Hebrides, and, in addition, an unknown 

 original thickness of White Bed. It is rather interesting to note 

 that the White Bed, which is the sole representative at Burton of 

 the time-interval between the Bed Bed and the Astarte Bed, 

 should not be found separating them, and has only been preserved 

 by an accident. 



Bearing on the remarks about the possible original thickness of 

 the White-Bed deposit, there is this to be said further: in the 

 Sherborne district the -pve- Garantiana zones of the Inferior 

 Q. J. a. S. 'No. 312. 2 G 



