part 4] JUEASSIC chronology : LIAS. 259 



Dr. A. Morley Davies. To all I tender most grateful thanks. ^ 

 For the Yorkshire area and for the South-Western Counties I have 

 also had my own field experience — chiefly in the last-mentioned 



Part of the following pages may be regarded as an extension of 

 the principle applied with success to sundry brickyard exposures 

 along a line of countrj^ — the principle of unlike faunas. Different 

 contiguous exposures show faunas of partly or wholty unlike f acies : 

 therefore their deposits are partly or wholly heterochronous — find 

 the sequence. In wider extension three are obviously other factors 

 for consideration — agency of deposition, difference of basin, con- 

 stituents of the fauna — like must be compared with like. But the 

 following pages show, it is hoped, that the method of work may be 

 extended and justified ; and that it may answer both wa^'-s — for, as 

 a proved sequence shows the meaning of dissimilar faunas, so dis- 

 similar faunas give reason for expecting heterochronous deposition. 



It is fairly evident that geologists have been using the principle 

 of dissimilar faunas in separating strata, without recognizing it — 

 much as M. Jourdain had Avritten prose wiihout knowing the fact. 

 But it is not proposed to pursue the matter further now — it 

 deserves a stud}^ to itself — and perhaps a terminology ! 



II. Yeoyiliax and Whitbia?s. 



- The whole of the Yeovilian and a considerable part of the Whit- 

 bian are unrepresented b}^ strata in the Scottish isles : a deposit, 

 about 75 feet thick in Baasay, which I term the Dun Caan Shales, 

 corresponding in date to the 'poat-i^ioorei part of the Bridport 

 Sands of South Dorset ^ (early Aalenian), rests non-sequentially on 

 the Raasay Ironstone, which is of the date of the lower part of the 

 Whitby Alum Shale — 'post-f a I cij'erum hemera. This non-sequence 

 represents a break of about 700 feet of strata on the evidence of 

 various English localities ^ ; and may prove to be very much more 

 when Continental dejDOsits come to be analysed with the same 

 detail. 



The chronology of the Yeovilian and Whitbian has been already'' 

 rather fully particularized in the last two communications men- 

 tioned in the list of works on p. 258 ; but there is reason to suppose 

 a fuller faunal sequence in the former, and (as regards the latter) 

 it seems desirable, on the ground of palafontological uniformity, 

 to class as early Whitbian the bed or beds hitherto regarded as 

 latest Domerian, see Summary, pp. 275, 276. 



The attention of investigators should be particularly directed 

 to the sequence of strata on the Whitbian-Domerian border-line — 



^ To Mr. Tutcher my thanks are due for his careful photographic ilhistra- 

 tions to the Pala8ontoh)gical Appendix. 



- Q. J. G. S. voL Ixvi (1910) pp. 60, 61, 64. 



■^ Ibid. vol. lix (1903) p. 456 ; ' Geol. AVhitby ' 2nd ed. Mem. Geol. Surv. 

 1915, chap. V, p. 75. 



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