PRESTWTCH BRITISH AND FOREIGN TERTIARIES. 209 



M. Graves* not only includes the " Sables glauconieux and Lits 

 coquilliers" in this lowest group of Sables Inferieurs, but also the 

 overlying " Glauconie grossiere'* (which M. D' Archiac places wdth 

 the *'Calcaire grossier"), making of this latter division, together with 

 the 1st, 2nd, and 3rd of D' Archiac, one group ; and forming a second 

 of the divisions 4, 5, and 6 of D' Archiac. These he designates as the 

 two " Groupes des Sables glauconieux." 



M. Raulinf, on the contrary, takes the "Glauconie grossiere" 

 and the " Lits coquilliers" out of this group, and classes both as sub- 

 ordinate members of the Calcaire grossier. 



M. Charles D'Orbigny, in his " Tableau General," also makes 

 two groups of these lower French Tertiaries, — 1st. " Sables quartzeux 

 glauconiferes" (divisions 1, 2, and 3 of M. D' Archiac) ; 2nd. "Argile 

 Plastique;" but in a work;]; more recently edited by him, these 

 are further subdivided into — 1st. An upper group, consisting of, 

 — Glauconiferous sands ; sands, sandstones, and pebble beds ; lig- 

 nites, plastic clay, and fossiliferous conglomerates. 2nd. A lower 

 one of the lower lacustrine limestone and quartzose sands. 



M. Alcide D'Orbigny, who considers that the variation of the 

 fauna of all this period results only from variations in the^ depth and 

 saltness of the waters, forms of these six divisions his " Etage Sues- 

 sonien ou Nummulitique§", — his oldest Tertiary group. 



In Belgium, M. Dumont divides the lower Tertiaries into his 

 *' Upper and Lower Ypresian," and " Upper and Lower Landenian" 

 Systems ; whilst Sir Charles Lyell, combining and modifying the 

 classification of Dumont, Omalius D'Halloy, and D' Archiac, groups 

 them into Lower Nummulitic beds, London clay. Plastic clay and 

 sands, Glauconite and Tufeau of Lincent, and Marls and Glauconite 

 of Heers. 



In England I have placed the Lower Bagshot Sands at the base of 

 the Brackiesham series ; then, in descending order, the " London 

 clay," the "Basement-bed of the London clay," the "Woolwich 

 and Reading series," and the "Thanet Sands." 



It is curious and instructive to observe how, in each country, the 

 grouping of these lesser divisions of the Tertiary series has been 

 based upon local conditions. This is probably right and necessary 

 in establishing a local order of superposition, but it shows also 

 how much local phsenomena will modify, even at short distances, the 

 apparent relations which exist in nearly allied and synchronous 

 strata ; for it is on some actual predominating feature in the several 

 districts that the various groupings are based. Thus I have, from 

 the recurrence of like lithological characters and in the absence of 

 paloeontological evidence, referred the Lower Bagshot Sands to a 

 subordinate position with the Brackiesham series ; whilst, both from 

 organic remains and structure, the London clay is referred to a 



* Essai sur la Topographie Geognostique du Departeraent de I'Oise, 1847, 

 p. 174 ; an excellent local work to which we shall have constantly to refer. 



t " Patria," 1847, vol. i. p. 370. 



t Manuel de Geologic, 1852, p. 86 and 171. 



§ " Cours Eleraentaire de Paleontologie et de Geologic," vol. ii. p. 704. and 713. 

 VOL. XI. — PART I. Q 



