498 STEATIGRAPHICAL GEOLOGY 



sand, which were supposed to occur in regular upward succession. 

 As the palaeontology of these deposits was more carefully studied it 

 was seen that the " Lower Greensand " was a distinct stage, but that 

 the Upper Greensand was inseparable from the Gault, and was to a 

 large extent merely a sandy facies of the Gault. Hence it became 

 necessary to combine these two facies under one name as a single 

 stage. For this the name Selbornian has been chosen from the 

 village of Selborne in Hampshire, though the names Gault and 

 Greensand are convenient terms for the argillaceous and the sandy 

 facies respectively. 13 Where the stage is complete four zones can be 

 distinguished. These are : 



4. Zone of Pecten asper and Cardiaster fossarius. 



3. Zone of Schlcenbachia rostrata ( = inflata). 



2. Zone of Hoplites lautus and H. interruptus. 



1. Zone of Douvilleiceras mammillatum. 



Of these zones the lowest is always a sand, the second is generally 

 clay (i.e. Gault), the third may be clay or marl or malmstone or 

 sand, the fourth is always a " greensand " or sandstone. 



The typical development of the argillaceous facies is in Kent, 

 and the beds are well exposed at Folkestone, where the " Lower 

 Gault" consists of grey clays with several bands of phosphatic 

 nodules and a layer of such nodules at the base embedded in dark- 

 green glauconitic sand. The characteristic fossils are Hoplites 

 interruptus, H. splendens, H. lautus (upper part), Lima parallela, 

 Pinna tetragona, Inoceramus concentricus. Below and forming the 

 base of the stage are 5 or 6 feet of coarse green sand containing 

 Douvilleiceras mammillatus. 



The Upper Gaiilt or zone of Sch. rostrata consists of pale-grey 

 marly clay about 78 feet thick ; this includes a bed of dark-green 

 glauconitic sand in the upper part, and is separated from the 

 Lower Gault by a layer of phosphatic nodules. These beds yield 

 Schlcenbachia rostrata, Sch. Goodhalli, Pecten raulinianus, Tere- 

 bratula biplicata in the upper part, Schlcenbachia varicosa and 

 Inoceramus sulcatus in the lower part. 



Inland the thickness of the Gault becomes much greater and 

 is about 200 feet near Maidstone. This increase appears to be 

 chiefly in the Upper Gault, but it is not till we reach Westerham 

 that the so-called " Upper Greensand " makes its appearance at 

 the top of the Gault, and thence it thickens westward through 

 Surrey. This deposit is known in Surrey as "firestone" and " hearth- 

 stone," and in Hampshire as " malmstone " ; it is a greenish-grey 

 siliceous stone consisting partly of fine sand and glauconite grains, 

 partly of sponge spicules and globular colloid silica, and it passes 

 down into sandy marls which are clearly part of the Upper Gault. 



