1898] PROBABLE DEPTHS OF THE GAULT SEA 307 



In his book "The building of the British Isles" (5), A. J. Jukes- 

 Browne says with regard to this question, "The clays of the Lower 

 Gault seem to have been deposited in a shallow sea of 50 to 70 

 fathoms deep, which is about the depth of the sea between England 

 and Ireland, while the fossils of the Upper Gault of Folkestone 

 indicate a depth of 100 fathoms and upwards." From the foramini- 

 feral data for each zone of the Folkestone Gault to be referred to 

 subsequently, we obtain a mean depth for the Lower Gault (Zones 

 I.-VII. of Price) of 830 fathoms. In a similar way the L T pper 

 Gault (Zones IX.-XIII.) gives a mean depth of 866 fathoms. 



The following are the zones with their separate results and 

 points of interest : — 



Zone I. The Green-sand seam at the base of the Gault. — This is a 

 dark- coloured argillaceous green -sand. The included fossils are much 

 rolled and worn, and this particular deposit appears to have been 

 subjected to the prolonged action of currents. The depth obtained 

 by the evidence of the foraminifera of this bed appears somewhat 

 great, but can be accounted for by the fact of there being a later 

 foraminiferal fauna present, besides the assemblage of glauconite casts. 

 This mixed fauna also appears again in Zone XII. of the Gault 

 (formerly referred to as Zone XL green-sand seam). 



For Zone I. basal bed a possible depth of 750 fathoms is 

 obtained. 



At this horizon a single example of Hormosina globulifera was 

 found, and although usually occurring at greater depths, it is inter- 

 esting to note that Dr Goes records it from a depth nearly corre- 

 sponding with the determination given above. 



The samples of green- sand collected during the voyage of the 

 'Challenger' were taken from depths less than 900 fathoms, the 

 average being 449 fathoms. With regard to the hydrographical 

 distribution of green muds and sands, Messrs Murray and Eenard 

 observe in the volume on "Deep Sea Deposits," p. 240, that they 

 " would appear to form an interrupted band along many continental 

 shores at the upper edge of the continental slope." 



The usual sequence of the shallow to the deeper parts of the 

 areas occupied by terrigenous deposits is in the order of green-sand 

 (where conditions for its formation are favourable), green mud and 

 blue mud. In the case of the Gault this was followed by a semi- 

 pelagic deposit forming the grey marls of the Upper Gault. 



From the samples of green-sand obtained by the ' Challenger/ 

 one may refer, for comparison, to the green-sand, Station IV., 

 between Cape St Vincent and Gibraltar, depth 600 fathoms. 



Zone 1. 5 feet above the base. — A dark clay, greenish when wet, 

 bluish when dry. 



This is probably equivalent to the modern fine glauconitic muds. 



