V ol. 67.] ANNIVERSARY ADDRESS OF THE PRESIDENT. lxXVU 



to visualise and remember the leading phenomena of stratigraphical 

 geology, and to group together numbers of facts and inferences 

 with which it was previously most difficult to retain touch. But 

 this is not all that we owe to the authors and followers of the 

 method. Such maps systematise future research, they direct us to 

 the places where the discovery of new facts is desirable, and they 

 enable each new fact to be utilised in checking, establishing, or 

 modifying, conclusions expressed by provisional lines on the maps. 

 It was only natural that this branch should advance with rapid 

 strides in Britain, because this country has done so much detailed 

 work, not only in the study of its own constituent formations, but 

 also in the mapping of their extent of outcrop. There are other 

 reasons also of which we should not lose si^ht. 



(a) Methods of Investigation. 



In order to obtain what Dr. Marr has called the ' geogram ' of a 

 formation in its greatest perfection, we require to know the entire 

 extent of its variations, not only along its outcrop, but in that part 

 which is hidden from sight ; and we ought to be in a position 

 to infer the probable variations in that almost equally important 

 part- which has been destroyed by denudation. 



The study of the outcrop has in many cases proved of great 

 service — because important variations, such as those of the Lower 

 Jurassic rocks, have been detected and worked out along the line 

 of strike. But the outcrop evidence decreases in value whenever 

 it happens that denudation has left, as in the case of the Chalk, 

 an outcrop which follows a condition line or condition strip of the 

 area of deposition. 



It is here that, in my opinion, insufficient use has been made 

 of the ' isodiametric lines' which Prof. Hull drew attention to, 

 and utilised to bring out the south-eastward thinning of the 

 Mesozoic rocks. We look to the projected Memoir of the Geological 

 Survey on the thickness of the British formations, which Fox- 

 Strangways began and Mr. T. Y. Holmes is carrying on, to give us 

 information whereby it may be possible to draw such lines through- 

 out the British strata. We may be able to deduce from such lines 

 not only much information with regard to the laws of distribution 

 of ancient sediments, with consequent inferences on their sources 

 and boundaries, but also some indication of the nature and extent 

 of ancient denudation. It is even possible that the lines may be 

 found to bear relationships with contemporaneous sagging and 



