﻿Vol. 6l.~] ANNIVERSARY ADDRESS OP THE PRESIDENT. lxxix 



confess that these cases are merely suggestive. I discussed one of 

 them eighteen years ago in a paper read before the Cambridge 

 Philosophical Society, 1 and may be allowed to refer to it here. 

 The Stockdale Shales of the Lake District contain bands of dark 

 graptolite-bearing shales, interstratified with lighter mudstones in 

 which graptolites are rare or absent. Sometimes the alteration in 

 fauna is accompanied by change in the lithological characters of 

 the rocks, indicating that the faunistic change is not due to an 

 inherent tendency in the organisms to undergo variation. At other 

 times the faunistic change seems to be unaccompanied by any 

 noticeable difference in the lithological characters of the rocks, save 

 such as is produced by the absence of organic material therein. 

 This suggests that the changes which produced the succession of 

 faunas of the different zones were due to something which did not 

 in these cases affect the lithological characters of the rocks ; and the 

 only cause of which I can conceive is climatic variation in an area 

 sufficiently remote from land to prevent the change from affecting 

 the nature of the sediment supplied to the area. I had hopes of 

 working at this subject in greater detail, but have hitherto been 

 prevented from doing so. I believe that this work, in conjunction 

 with that suggested previously (namely, the attempt to ascertain 

 the variations in the proportions of the different species in successive 

 zones), would well repay anyone who undertook it. 



That the plane separating groups of rocks which are distin- 

 guished one from the other by differences in lithological characters 

 or organic contents is not always coincident with the plane which 

 divides a synchronously-formed group from one formed before or 

 afterwards is well known. Let us take note of a few examples. 



As regards lithological characters, we find illustrations among 

 the Cretaceous rocks of our islands. The Gault of the south is 

 represented in Northern Norfolk, Lincolnshire, and Yorkshire by the 

 Bed Chalk, so that anyone mapping these formations by lithological 

 characters would draw the basal line of the Chalk beneath rocks 

 of Albian age in the North-East of England, and beneath rocks of 

 Cenomanian age in the South of England. Again, we find glauco- 

 nitic deposits of Lower Greensand age in the North-East of England, 

 of Gault age in the South-West, and in Northern Ireland of an 

 age when parts of the English Chalk were being deposited. If, 

 therefore, we could trace the Cretaceous rocks continuously from 



1 ' On Homotaxis ' Proc. Camb. Phil. Soc. vol. vi (1887) p. 74. 



