Revieivs — J. E. Marr's Stratigraphical Geology. 85 



their distribution in time and space varies greatly in most cases. 

 We have to bear in mind that the organic remains to be found in 

 limestones, clays, and sands of the same age will dijSer in a marked 

 degree. Nevertheless, there are to be found certain widely diffused 

 forms — trilobites, graptolites, brachiopods, belemnites, ammonites, 

 and others — which serve to act as indices of age. Species of some 

 of these fossils have been found to occur in definite sequence, as 

 definite as the sequence of strata, but irrespective of the kinds of 

 sediment. Consequently there have been established certain 'zones' 

 which are characterized either by one or more particular species or 

 by assemblages of fossils. They are clearlj' palaeontologioal horizons, 

 for they are not lithological divisions of the strata. In a conformable 

 series no one knows the precise limits of range of any one species. 

 In useful diagrams given by the author (p. 62), he shows how each 

 species may have a varied range in time in a formation or series of 

 strata. No doubt each species has its definite chronological and 

 geographical limits, although we can never know exactly what these 

 are. The author remarks that, " strictly speaking, the term zone 

 (a belt or girdle), when applied to distribution of fossils, should 

 refer to the belt of strata through which a fossil or group of fossils 

 ranges." We do not quite agree with the author in this definition. 

 Zones of course must be identified by fossils and not by the strata ; 

 and therefore they serve to indicate periods of time rather than 

 masses of sediment. We could hardly speak of a zone which is 

 represented partly in Upper Greensand, partly in Red Chalk, and 

 partly in Gault Clay as forming a belt of strata. A zone may be 

 represented, as the author points out, in strata 8 inches thick or 

 5,000 feet thick ; but the true belts of strata are such as he well 

 represents in his diagram, p. 175, and we wish he had given mora 

 of these stratigraphical figures. 



The author devotes a brief chapter to geological maps and 

 sections, and points out the great advantage of drawing longitudinal 

 sections to a true scale. In addition to the ordinary geological 

 maps which depict the stony structure of the country, he would like 

 to have also a set of chronological maps founded on the sequence of 

 fossil organisms. We wonder who could construct such zonal maps. 

 It is possible that over small areas, by aid of numerous excavations 

 and long-continued fossil-collecting, something might be done to 

 indicate in a rough way the paljeontological divisions. We should 

 like the author to map (irrespective of any lithological aid) the 

 Upper Cretaceous subdivisions — the Albian, Cenomanian, Turonian, 

 and Senonian, to use the chronological terms which are sometimes 

 employed. We doubt, indeed, if any field-geologist would find these 

 terms so useful on the ground in this country as Gault and Upper 

 Greensand, Lower, Middle, and Upper Chalk. 



The author very properly points out that " The establishment 

 of a classification on palseontological lines by no means does away 

 with the necessity for local classifications on a lithological basis." 

 Each country is apt to frame its local classification on stratigraphical 

 and lithological grounds, hence when local terms from other 



