Yol. 50.] ANNIVERSARY ADDRESS OF THE PRESIDENT. 79 



mainly of dark graptolite-bearing shales, which alternate with 

 lighter-coloured mudstones entirely devoid of graptolites except 

 where they pass into the adjacent graptolitic shales. The Upper 

 group (Browgill Beds) is nearly three times as thick as the lower 

 one, having probably been formed much more rapidly, and conse- 

 quently of less importance ; it consists chiefly of green and purple 

 shales with interstratified grit-bands and a few insignificant seams 

 of dark graptolite-bearing shales. Although there is absolute 

 conformity between the lowest beds of the Stockdale Shales and the 

 highest beds of the Ashgill Shales, the palseontological break is 

 complete, and it is at this point that the Authors draw the line of 

 division between the Ordovician and Silurian systems. 



One very singular fact in connexion with this enquiry, having a 

 wide bearing on stratigraphical palaeontology in general, is the 

 remarkable recurrence of graptolitic and non-graptolitic beds so 

 characteristic of the lower series. The subject of recurrent faunas 

 is well-known in several formations, and in some cases is more or 

 less due to changes, which partly indicate their nature by a differ- 

 ence in the character of the sediment. The Authors, and especially 

 one of them, who has ably discussed this subject before the Cam- 

 bridge Philosophical Society, are disposed to consider it due to 

 climatic changes in the present case, which is certainly one of the 

 most remarkable to which the attention of palaeontologists has been 

 drawn. There is another question in connexion with these beds to 

 which they also draw attention, viz. Are the graptolites wholly 

 absent from the trilobite-bearing mudstones, and vice versa ? The 

 usual difficulty which attaches to the proving of a negative is 

 naturally felt, though they conclude that there was, possibly, com- 

 plete migration in some cases, whilst in others the forms may have 

 lingered on in diminished numbers during the period that was 

 unfavourable to their existence. In this latter case, they suggest 

 that such a lingering on would be admirably qualified to bring about 

 that variation in the creatures which would account for the marked 

 contrast between the fossil contents of beds separated by only a few 

 feet of intervening rock. 



The Authors claim that the most important result of their 

 researches is the additional evidence they have furnished of the 

 value of graptolite-zones as a means of comparison of Lower Palaeo- 

 zoic rocks of distant areas. It is true that a suggestion was made 

 to the effect that possibly the application of graptolitic as against 

 trilobitic verniers might not produce the same results in the way of 



