436 IF. D. Lang — Carboniferous Zones marked by Corals. 



zones into sub-zones named after species of Corals, Braehiopods, and, 

 in one instance, a Lamellibranch Mollusc. 



Kow all Vau^lian's zones except two are named after Coral 

 genera ; and for these dissenting zones there ai'e corresponding Coral 

 equivalents, since the heraera of Lithostrotion nearly coincided with 

 that of the Brachiopod Semimda, and the hemera of the Coral Canmia 

 with that of Syringothyris. Again, the only sub-zones named after 

 Corals are the two subdivisions of the topmost zone. It is possible, 

 then, with the Corals used by Vaughan as tlie indices of his zones and 

 sub-zones to make six consecutive divisions of the Lower Carboniferous 

 rocks. The main part of the exhibit demonstrates these six sub- 

 divisions, namely, from below upwards, the zones of Cleistopora, 

 Zaphrentis, Caninia, Lithostrotion, and the sub-zones oi Dihunophyllum 

 and Lonsdalia florifonnis of the Dibunophyllmn zone, each with its 

 distinctive letter for abbreviated reference, K, Z, C, S, Dj^, and Dg ; 

 and gives examples of the Corals after which the zones are named, 

 with, in some instances, another species found with the zonal Coral, 

 and diagrams showing the zonal Corals' structure. 



Since it was in the gorge of the River Avon at Bristol that Vaxighan 

 founded these subdivisions of Lower Carboniferous rocks, photo- 

 graphs of this typical section have been placed at the sides of the 

 case, and alongside them diagrams showing the limits of the zones 

 traversed as the right bank of the Avon is followed from north to 

 south.' 



It is to be expected in most cases where stratigraphical divisions 

 are founded upon a single dominant species or genus that it will be 

 possible to applj' the same divisions to other areas with a strictness 

 tiiat is only inversely proportional to the distance traversed. Thus 

 Vatighan's zones, though found to hold good in the main, require 

 some modification where the Lower Carboniferous rocks of other 

 areas in the British Isles have been examined." It is not at present 

 possible to show these modifications, and this exhibit is confined to 

 the zones of the typical area which has served as a basis for 

 subsequent work. 



Two other points need emphasis in connexion with Vaughan's 

 zones. The difference between the time during which a species 

 existed and its hemera has already been noted. Again, the sequence 

 of zones does not necessarih^ imply a phylogenetic series.^ Thus, 

 while Cmiinia is almost certainly a direct derivative of Zaphrentis, it 

 ])robably does not lead on to Dihunophyllum ; and Cleistopora, even 

 if a Coral, is certainly not a radical of the Rugose genera of the 

 liigher zones. It is thought that there was much migration of Coral 



' Professor S. H. Eeynolds kindly lent his negatives of the photographs 

 published in Vaughan's paper, " The Carboniferous Limestone Series (Avonian) 

 of the Avon Gorge," Proc. Brist. Nat. Soc, ser. IV, vol. i, pt. ii, pp. 74-168, 

 1906, from which the exhibited photographs were printed. 



- e.g. in North- West Ireland, see "The Lower Carboniferous Succession at 

 Bundoran in South Donegal ' ' in The Geology of Parts of North- Western 

 Ireland, issued by the Geologists' Association for the August excursion, 1912, 

 p. 9. 



^ See Carruthers, ' On Coral Zones in the Carboniferous Limestone": 

 Geol. Mag., Dec. V, Vol. VII, p. 172, 1910. 



