﻿Vol. 64.] SUBDIVISIONS OF THE CHALK OF TRIMINGHAM. 



403 



Yariation of dip by using \ for dips estimated not to exceed 20°, 

 ^ for those estimated to exceed 50°, and ^ for intermediate 

 dips. 



In describing the three great blocks in which the Chalk now 

 occurs, it will be convenient to take first that (A) which terminates 

 in the South Bluff — the only bluff now existing, as the much dis- 

 cussed North Bluff finally disappeared in February 1907, except for 

 a small outlier of Grey Chalk which still remains. This block forms 

 an east-and-west anticline. The lowest beds exposed in it are four 

 beds of White Chalk appearing in a narrow oval close to the bluff, 

 which are marked (a) on my map (PI. XLYII). The line of flint (6) 

 which separates the upper two of these beds is readily recognizable 

 by the great size of the individual flints of which it is formed, these 

 being far larger than the flints in any other line of flint seen at 

 Trimingham. Possils are fairly abundant in these beds ; they have 

 been carefully collected; and, although the total area exposed is 

 relatively small, I have found the following significant forms; — 



Serpula canteriata (four-angled 



variety), very rare. 

 Terehratula sex-racliata, common. 

 Terebratulina Gisei *, very rare. 

 Terehratuliria gracilis ^, rare. 



hcBraglohularis, very common. 

 JPo7^osph(Bra nuciformis, rare. 

 Echinoconus Orbignyanus -x-, rare. 

 Pentacriuns Agassizi -!f. 

 Pentacrinus Bronni *. 

 Pentacrinus sp. nov. 



The forms marked with an asterisk are apparently confined to the 

 Trimingham Chalk: they occur throughout it, and may be taken as 

 the characteristic fauna of a zone comprising all the Chalk exposed 

 at Trimingham. They are all quite rare in these beds. Small 

 Echinoconi are abundant, but the great majority appear to be E. ah- 

 hreviatus. This fact strongly suggests that these beds are very 

 near the base of the zone, and this is confirmed by the single 

 specimen recorded as Pentacrinus sp." nov. This is identical with 

 the Pentacrinus abundant in the Overstrand erratics, which contain 

 none of the peculiar Trimingham forms and must therefore be 

 referred to the zone of BelemniteIJa mucronata. The near approach 

 of these beds to the latter zone is also suggested by the occurrence 

 of Terehratula sex-racliata and the abundance of Porosplio'.ra glohu- 

 laris. It will doubtless surprise those who are familiar with the 

 whole of our Senonian Chalk to find Porosphcera invested with 

 significance, but the behaviour of the genus is remarkable. In the 

 Senonian, and especially in the two upper zones, the regular forms 

 P. arrecta, P. glohularis, P. nuciformis, P. galeata, Stolley ^ [= -^« 

 patelliformis, Hinde, a name which must give place to that assigned 

 by Dr. Stolley, whose description and figure are quite recog- 

 nizable], and P. pileohis are widely and freely distributed, while 

 irregular forms are rare. In the Trimingham Chalk irregular forms 

 are abundant ; while, of the regular forms, I have never found 

 P. arrecta or P. galeata, I have but one specimen of P. j^Heolus, 

 horizon uncertain, and P. nuciformis only occurs in a few very 

 poorly characterized specimens in the beds under consideration and 



1 Mitth. Min. Inst. Univ. Kiel, vol. i, pt. iv (1892) p. 276 & pi. ix, fig. 8. 



