320 A. Stra/ian — CMoritic Marl, Dorset. 



The thickness of sand between the fault and what may be taken as 

 the base of the Chalk amounts to 15 feet. 



The following list of fossils includes those collected in April, 1901, 

 and those obtained previously by Mr. Hill. They prove that the 

 whole of the sand should be referred to the Chloritic Marl. The 

 identifications are by Mr. E. T. Newton. . 



CHLOEITIC MARL, MUPE BAY. 

 Scattered through the Bed. 



Terebratula squamosa, Mant. Solaster subglobosa, var. alius, Ag. 



Cidaris Boiverbanki, Forbes. Holasfer suborbiciilaris ? Defr. 



Echinoconus castanea, Brongn. Flocoscyphia ? 

 Solaster subglobosa, Leske. 



From Nodule-bed at base. 



Helicoceras, sp. Cardita, sp. (? tenuicosta). 



Columbellina ? Pleuromya (Pa^iopcea) . 



Turritella, sp. Cytherea plana, Sow. 



CueullcBa carinata ? Sow. Plicatida, sp. 



Cucullcsa mailleana? D'Orb. Exogyra, s^. 



Isocardia, sp. Polyzoon on Lamellibranch. 



Trigonia, sp. Rhynchonella (near to grasiana). 

 Cardium, sp. 



A section at the top of the cliff (referred to on p. 151 of the 

 Memoir on the Isle of Purbeck) shows that the Chloritic Marl 

 maintains an abnormal thickness for 200 yards at least west of 

 the section on the beach. It is there succeeded downwards by 

 a considerable thickness of Upper Greensand, in which, however, 

 no representative of the Chert Beds could be found, and which 

 presumably belongs to the lower part of the formation. Two 

 explanations for this sequence may be offered : firstly, that the 

 fault which throws Chloritic Marl against Gault on the beach does 

 not follow the bedding precisely, but cuts obliquely through the 

 Upper Greensand, so as to throw Chloritic Marl against that forma- 

 tion westwards ; secondly, that the upper part of the Upper Greensand 

 suffered erosion before the deposition of the Chloritic Marl. 



The first explanation is supported by the fact that the section on 

 the beach places the existence of a fault beyond doubt, and shows 

 also that the fault is approximately parallel to the highly inclined 

 beds. In favour of the second explanation it may be urged that the 

 abnormal thickness and character of the Chloritic Marl point to the 

 Upper Greensand having undergone erosion. The phosphatic casts 

 of fossils, though but little water- worn, are fragmentary. While, 

 therefore, they have neither travelled far nor been rolled upon 

 a beach, they are not in their original matrix. The absence of 

 any fragments of chert among the nodules in the Chloritic Marl 

 appeared to me to point to the erosion of the Upper Greensand 

 having been but slight, but in explanation of this Mr. Hill suggests 

 that the chert-nodules were formed at a later date, and that the beds 

 were all soft when the erosion took place. Tor myself I am inclined 

 to think that the segregation of nodules followed as a rule closely 

 upon the deposition of the sediments in which they occur. 



