ANNIVERSARY ADDRESS OF THE PRESIDENT. xli 



have survived from the prodigious numbers (if all were named, pro- 

 bably more than 500 species) found in the Caradoc and Bala beds ; 

 four are peculiar; twenty pass into the Upper Llandovery rocks, 

 thirteen of which also pass into the Wenlock shale. 



While therefore a fraction more than a half of its fossils are found 

 also in Lower Silurian rocks, the disappearance of so large a proportion 

 of Caradoc species proves a very great change of conditions. Con- 

 sidering the relative numbers in the two formations, it is too much 

 to suppose that the older fauna was destroyed by the invasion of a few 

 new species from another area, although, if the beds be conformable 

 to each other, this is probably an obvious though not the most likely 

 explanation. The suspicious unconformity between Caradoc and 

 Llandovery beds in South Wales points in the direction that in reality 

 there is a gap, due to upheaval and denudation, unrepresented by 

 strata, between the two formations, and that on re -submergence only 

 a few of the ancient Caradoc forms survived, to mingle with newer 

 forms at first almost equally limited in number. 



The Upper Llandovery beds have yielded as yet about sixty species, 

 twenty of which occur in the Lower Llandovery rocks. Eight 

 species belonging to the latter have disappeared, and only twelve 

 Upper Llandovery forms are known in the Caradoc Sandstone. 



The absolute unconformity of the Upper Llandovery beds on all 

 below, coupled with a great change of species, is another remarkable 

 coincidence, and is clearly connected with a lapse of unrepresented 

 time ; for the Lower Silurian strata were, in our area, in places me- 

 tamorphosed, intensely contorted, upheaved, and extremely denuded 

 before the deposition of the Upper Llandovery beds began. Such 

 events involve the lapse of a period of time (unrepresented by strata) 

 which it is almost impossible to exaggerate ; and I believe that we 

 see the result in the loss of old species and the appearance of new, 

 in proportions comparable to the differences between the fossils of 

 the newest Miocene and oldest Pliocene beds and the life of the 

 present day. The change in this respect is, however, far less both in 

 genera and species than that which took place between the Lingula- 

 and Tremadoc and Llandeilo beds ; and therefore, possibly, the smaller 

 changes represent shorter periods of stratigraphically unrepresented 

 time. 



• Wenlock Shale, Sfc. — If we now examine the relation of the Wen- 

 lock Shale to the Upper Llandovery beds, we shall find that, out of 

 fifty-six species, twenty-eight, or one -half, pass upwards into the 

 former, which frequently overlaps the Llandovery beds in such a 

 manner as to leave no doubt of an unconformity that must again 

 indicate a period of unrepresented time, after which we have the 

 vast development of life of the undoubted Upper Silurian epoch, 

 during which 5000 or 6000 feet of strata were deposited in a period 

 of apparently slow and steady depression. 



Furthermore it is evident, from the sandy character of most of 

 the Llandovery strata, from the conglomeratic nature of part of the 

 upper beds (derived from the waste of the Lower Silurian rocks), 

 from the comparative thinness, local character, and repeated uncon- 



vol. xix. d 



