Reviews — Jukes-Browne 8^ Hill — Gault and U. Grcensand. S-j 



nifiy be regarded as in some measure introductory to the completed 

 monograph on the Cretaceous System. In it the author places on 

 record some of the conclusions drawn from a study of the zones in 

 this system, " especially with respect to the proper conception of 

 a zone, the use of an index species, and the limitations which must 

 be placed to the zonal method." To give a succinct explanation of 

 a zone is by no means easy; the author says that "perhaps it may 

 be defined as a band of sedimentai'y material within which certain 

 species ai-e either restricted or are specially abundant, and during the 

 formation of which certain species acquired their greatest exuberance 

 and their greatest geographical extension. More than this, however, 

 is implied by the modern idea of the term zone, for a zone is only one 

 of several successive zones ; it is not merely a specially fossiliferous 

 band in a thick mass of sediment, but is a subdivision of such a mass 

 or group of beds ; such a group being generally divisible into two, 

 three, or more zones, one succeeding another." The above definition 

 seems to us open to much criticism, which, however, cannot be 

 entered on here ; we should prefer the shorter definition of Mr. J. E. 

 Marr, here quoted : " Zones are 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." 



A general account of the Gault and Upper Greensand (Selbornian) 

 is given in Chapter iv, and it is claimed that the clays, marls, sands, 

 and sandstones of this Selbornian stage fall naturally into three 

 groups or sub-stages : (1) Lower Gault; (2) Upper Gault and Devizes 

 Beds ; (3) Warminster Beds. 



Hitherto it has been usual in England to consider the clayey beds 

 containing Ammonites interruptns as the base of the Gault, and 

 the underlying sandy beds as belonging to the Lower Greensand. 

 In the uppermost beds of these lower sands at Folkestone and in 

 three other localities in the South of England Ammonites mammillatns 

 has been met with, whilst in France the same species occurs in 

 a zone of fossiliferous sandy beds in association with Am. interruptns, 

 and by French geologists these beds are included in the Albian as 

 part of the basement bed of the Gault. The author considers this 

 will justify placing the sands with this fossil as the base of the 

 Gault in this country, although it has never been found hero 

 associated with Am. interruptns. 



The zone of clays with phosphatic nodules at its base, containing 

 Am. interruptns, forming bed 1 of Mr. Price, is about 10 feet iu 

 thickness at Folkestone and from 20 to 50 feet in the Midland 

 Counties. The upper part of the Lower Gault, which includes 

 Price's beds 2-7, is placed in the zone of Am. lautus. It can be 

 distinguished near Devizes, and is believed to form the larger part 

 of the Lower Gault in Oxfordshire and the adjoining counties. 

 The thickness of the Lower Gault in different parts of tlie country 

 varies between 34: feet and 200 feet, but there is much difficulty in 

 determining with certainty where a line can be drawn between the 

 Lower Gault and the Upper in many areas. 



The next division comprises the Upper Gault and Upper Greensand 



