A. J. Jukes-Browne — Zones and Maps. 219 



to be quite unaware that this is the accepted view of a zone ; 

 he may, of course, still maintain that he is right and that the 

 authorities I have quoted are wrong, but if so, something more 

 satisfactory in the way of argument will be expected from him. 



Why, again, should the reviewer deem it impossible to produce 

 such maps as Mr. Marr has expressed a desire to see — maps in 

 which the colours should rigidly signify stages or divisions founded 

 on palgeontological evidence, and not in any way on the lithological 

 facies of the rocks. He intimates that the Cretaceous stages of 

 Albian, Cenomanian, etc., could not be mapped in this way, but 

 I have probably had as much experience of these strata as the 

 reviewer, and I will undertake to say that they could be mapped by 

 palasontological evidence, though of course the surveyor would have 

 to know his fossils well to start with. 



It is a mistake to suppose that the only kind of geological map 

 which can be of any use is one showing the different kinds of 

 rock which occur in a given district. That is doubtless the kind 

 of map which is best suited for economical purposes, and as such 

 it is the kind of map issued by our Geological Survey and by the 

 Service de la Carte Geologique de la France ; but that is, after all, 

 only one kind of geological map. There is certainly another kind, 

 more strictly scientific and just as truly stratigraphical ; this may 

 be called a chronological map, though a better term might be found 

 for it, and the smallest division shown by colour on such a map 

 would be a stage, such as Bathonian or Turonian. 



There would be no more difficulty in producing a series of such 

 maps than there is in drawing a line between the Gaultand the Upper 

 Greensand in Surrey and Hampshire, a task which I commend to 

 the reviewer, and to any geologists who still believe in the exceeding 

 usefulness of such divisions as Gault and Upper Greensand. 



As a matter of fact, I am informed by my friend Professor Barrois 

 that the Geological Service of France is actually now issuing such 

 maps; sheets on a scale of -3— dVo-o> with a system of coloi'ation by 

 stages and showing what we will call the 'solid geology' only, 

 without obscuration by superficial deposits. 



When, therefore, Mr. Marr asserts that " for theoretical purposes 

 two sets of chronological maps, one founded upon lithological 

 differences, the other upon differences of fossil organisms, would 

 be extremely valuable," ^ he should be commended for his foresight, 

 and he will doubtless be glad to know that in one country, at least, 

 the need for such maps has not only been felt but met. 



The mapping of zones would be a more difficult matter, though in 

 many cases of well-established zones it could doubtless be done at 

 once ; but it is safe to say that in process of time, when the zonal 

 succession in every formation has been more fully studied and fairly 

 well established, the larger zonal divisions or groups of sub-zones 

 could be distinguished on such maps, either by tints of the 'stage' 

 colour or by a system of lettering. 



1 " Principles of Stratigraphical Geology," pp. 15 and 87. 



