« CllLORITIC MARL. 79 



dance of organic remains. He considers that the fact that young 

 shell-fish are frequently attached to iheir exterior, and that the 

 material now filling the so-called perforations seems to have been 

 introduced after the formation of the nodule, leads to the con- 

 clusion that the nodules were formed on the sea-bottom contem- 

 poraneously with the deposition of the Chalk, and formed at one 

 time a suitable home for some kind of boring animal. There is no 

 structure in the nodules that points especially to sponges as having 

 been their origin. On the other hand Mr. Hill remarks that their 

 occurrence in strata exceptionally rich in fossils is suggestive of 

 their having resulted from the decay of organic matter. 



The finest sections of the Chalk-with-flints form the precipices 

 of Scratchells Bay and Culver CUfF. In each case the lines of 

 flint enable the eye to follow the bedding from a distance, and to 

 take in at a glance the regularity of the great curve in which 

 the Chalk rises from beneath the Tertiary, and arches over the 

 Secondary formations (see Section, Plate I.). 



The thickness of the Upper Chalk can be arrived at by calcula- 

 tion only. Lines of section have been plotted across four different 

 parts of the central line of Downs, <iiving a mean thickness of 

 ],350 feet, or rather more than the 1,017 feet assigned to this sub- 

 division by M. Barrois. 



In describing the sections, it will be convenient, after dealino; 

 with the Chloritic Marl, to take the three sub-divisions of the 

 Chalk together, for it generally happens that the same pit, or 

 group of pits, provides sections of parts of all of them. 



The Chi.oritic Marl. 



The Chloritic Marl received its name from the abundance of 

 grains o£ green colouring matter in it, formerly regarded as chlorite, 

 but now recognised as glauconite. Although a calcareous deposit, 

 it is remarkable for the number of phosphatised casts of Ammonites, 

 Turrilites, and other fossils it contains. These were at one time 

 worked for phosphoric acid on St. Catherine's Down, but the 

 attempt was soon abandoned. The Chloritic Marl varies in thick- 

 ness, being 13 feet at Compton Bay, IH feet at Brook, 7 feet at 

 St. Catherine's, 8| feet above Ventnor, and 15 feet at Culver Cliff. 

 The variation is perhaps accounted for by the fact that no definite 

 line can be traced between it and the Chalk Marl above. The 

 lower beds of the Chalk Marl not only contain an abundance of 

 sand and glauconite, but sometimes also rolled phosphatic nodules, 

 not distinguishable from those in the Chloritic Marl (see section 

 at Compfon Bay, p. 83). It is generally diificult to decide at what 

 exact horizon the proportion of sand in the rock falls so far below 

 the proportion of calcareous matter as to justify the bed being 

 referred to as chalk. 



The relations of the Chloritic Marl have been discussed by 

 Messrs. Barrois, Parkinson, Meyer, Jukes-Browne, and others. 

 By M. Barrois it was grouped with the Chert Beds and the 



