402 SCIENCE PROGRESS 



Rock of the Midlands. Its roughness is due to the presence 

 of innumerable comminuted fragments of Inoceramus mytiloides. 



Zone of Holaster subglobosus. 



The Grit Bed ends abruptly, its under-surface being flat and 

 smooth, so that there is a sharp line of demarcation between 

 it and the underlying bed. This is the junction of the Turonian 

 and Cenomonian, of the Middle and the Lower Chalk. 



The top of this consists of a smooth, dark, greenish-blue 

 flaky marl, a striking contrast to the overlying Grit Bed. This 

 bed of marl is about 4 ft. thick, and is characterised by the 

 frequent occurrence of a large belemnite known as Actinocamax 

 plcmis. This species is common in this band, which is con- 

 sequently known as the Belemnite Marl, and dies out in the 

 firmer chalk beneath. The Belemnite Marl is very persistent, 

 and can be followed throughout the Chalk area even into 

 Yorkshire, where it is present as a narrow black carbonaceous 

 seam. 



Beneath it we have a considerable thickness of massive, 

 dense, compact, yellowish marly chalk, the true chalk of 

 Holaster subglobosus. This is 142 ft. thick, and is good ground 

 to sink through, as it is firm without being too tough, and 

 quite dry. Fossils are rare, but a few sharks' teeth occur, 

 and occasional specimens of the name-fossil. 



Zone of Ammonites varians. 



This zone has several subdivisions. At the top of the 

 zone is the Cast Bed, a band only about 2 ft. thick rich with 

 fossil remains. The forms to which we have grown accustomed 

 in the Chalk now disappear, for though we find the familiar 

 genera the species are different, and the whole assemblage 

 more resembles the fauna of the Gault. 



Beneath the Cast Bed we have a dark blue-grey, impervious 

 chalk, rich in marl, generally soft, but with numerous very 

 hard bands. Towards the base there are a series of ihin stony 

 beds full of sponge remains : this is sometimes called the 

 sub-zone of Plocoscyphia labrosa, and is about 10 ft. thick. 

 Beneath these hard reefs there is a remarkable and very 

 distinctive belt of very dark green arenaceous marl. This is 

 the Glauconitic Marl, formerly incorrectly termed the Chloritic 

 Marl, and confused with the Upper Greensand. It is in reality 



