CHALK. 93 



from Compton Bay to Freshwater and in the cliffs known as the 

 Nodes and the Main Bench. 



Zone of Micraster coranpuinum. 



This zone has furnished but few fossils ; and differences in 

 fauna were not therefore relied upon by M. Barrois in making 

 this sub-division of 500 to 550 feet of chalk. He correlates it 

 with the two divisions estabhshed by INIi'. AVhitaker in the Chalk 

 of the Isle of Thanet, namely the Margate Chalk above, and the 

 Broadstairs and St. Margaret's Chalk ])elow. In this lower 

 division in the Isle of Thanet he has obtained many specimens of 

 Micraster coranguinum, and in the upper, a great abundance of 

 Belemnites verus, Miller, Marsujntes Milleri, Mant., M. ornatus, 

 Miller, which, according to M. Hebert, are characteristic of the 

 upper part of the zone of Micraster coranguinum. The upper or 

 Margate zone also contains but few flints, while the lower or 

 Broadstairs zone contains a great number. These two zones he 

 considers to be recognisable in the Isle of Wight. To the Margate 

 zone he attributes the chalk of the great ({uarry on Arreton Down, 

 and of that to the east of Mersley Down; while the Broadstairs 

 and St. Margaret's type is seen in the small quarry of Bowcombe 

 Down. 



Zone of Belemnitella. 



The great quarry to the north of Shalcombe Down shows, in 

 the lower part, white chalk with many large black flints, and, 

 in the upper part, softer chalk with smoke-grey flints. These 

 correspond respectively to the zones known in France as those of 

 Belemnitella quadrata and of B. mucronata. There are many 

 quarries along the north side of the Downs, all in the zone of 

 Belemnitella, but the deepest only reach the horizon of B. quadrata. 

 The flints of the zone of B. mucronata are often grey as at 

 Shalcombe and the Needles, but sometimes black, as at Alvington 

 and Mottistone. In the upper part of the lower zone (that of 

 B. quadrata), Magas jnimilus is abundant. The united thickness 

 of these zones of Belemnitella is 260 feet- 



The junction of the Belemnitella zone and the zone of Micraster 

 coranguinum may be observed on Arreton Down, but, except in their 

 palaeontological characters, there is little difference between them. 

 They are distinguishable only by the relative abundance of flints 

 in the Belemnitella zone, and their almost entire absence in the 

 upper part of the Micraster zone. 



M. Barrois alludes also to the road-cutting near Apes Down, 

 which extends for some three hundred yards along the junction 

 of the Chalk and Plastic Clay. The section has now become some- 

 what obscured by talus and vegetation, but the contrast between 

 the red clay of the north, and the white chalk of the south side of 

 the road, is still sufficiently striking. 



