218 A. J. Juices- Broivne — Zones and Mops. 



To select a few out of many authorities the following may be 

 mentioned : — 



(1) Professor Lapworth states that stages or groups of strata 

 " are divisible into zones or minor groups of strata." See " Intro- 

 ductory Textbook " (Page & Lapworth), 1888, p, 125, and " Inter- 

 mediate Textbook," 1899, p. 155. 



(2) Kayser & Lake, who give the same definition. (" Textbook 

 of Comparative Geology," 1895, p. 6.) 



(3) Sir A. Geikie—" Textbook of Geology," first edition (1882), 

 p. 635, and third edition (1893), p. 678 — defines a zone as a bed, or 

 a limited number of beds, characterized by one or more distinctive 

 fossils. 



(4) Mr. S. S. Buckman, who remarks : " I take it that of late years 

 the stratum or strata characterized by an assemblage of organic 

 remains, more or less peculiar thereto, has been regarded as a zone." 

 (Quart. Journ. Geol. Soc, xlix, 1893, p. 481.) 



(5) Mr. J. E. Marr, who defines zones as "belts of strata, each 

 of which is characterized by an assemblage of organic remains, of 

 which one abundant and characteristic form is chosen as an index." 

 (" Principles of Stratigraphical Geology," 1899, p. 68.) 



In view of this consensus of recent opinion it is surprising to find 

 that the reviewer of Mr. Marr's book in the Geological Magazine 

 for February dissents from the definition just quoted, and says: 

 " Zones 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 " (op. cit., p. 85). 



This is not a very lucid statement, but the reviewer evidently 

 objects to a zone being regarded as a definite stratigraphical unit ; 

 he is attached to the old lithological method of classification, and 

 does not believe in mapping by fossils. But he should not find fault 

 with Mr. Marr for giving expression to the general belief of the 

 day — and, as we have seen, according to that general belief a zone 

 is a group of strata; further, as a zone literally means a girdle or 

 belt, it is correct to use that word in defining it. 



Again, if the same assemblage of fossils can be traced from 

 a Greensand area through a Gault area and into one of Eed Chalk, 

 does not that assemblage occupy a belt of strata ? The belt may 

 vary in thickness, but that does not matter ; it may not only pass 

 laterally from sand to clay, but it may in one district include both 

 sand and clay, and such variation does not make it any the less 

 a band or belt of strata. 



The zonal method, in fact, is merely a development of William 

 Smith's original method of identifying formations by means of the 

 organic remains which they contain. Just as one stage is identified 

 and distinguished from others by the assemblage of fossils it contains, 

 so in its turn is the zone identified by its fossils. The stage is not 

 a fauna, but a subdivision of a series or sj^stem of strata, and in like 

 manner the zone is a subdivision of a stage. The reviewer appears 



