PORTLAND KOCKS OF ENGLAND. 195 



98 feet, partly by the intercalation of new beds and partly by the 

 thickening of the rest. 



In the Portland-sand series the peculiar features of the island are 

 not repeated : the clay band at the top cannot be recognized ; and 

 the oyster-bed is certainly absent. Thus the whole might be de- 

 scribed as sandy marl with indurated bands. The parts, however, 

 of the series have characters by which correlation is made possible, 

 and a proof afforded that much more is here visible. 



Thus ISTo. 11 consists of 39 feet of the liver-coloured mixture, 

 which indurates into two blocks of sandy cement-stone. Above the 

 upper block should be the clay ; but it is scarcely less sandy than 

 the rest. Ammonites hiplex and Mytilus autissiodorensis are here 

 abundant. There are also Pecten solidus, Trigonia incurva, T. Pellati, 

 and another species more nearly allied to T. gibhosa. 



No. 12 shows no particular change at the top ; it is aU dark, 

 hard, sandy marl, with numerous small cement-stones, lying in bands, 

 and the two lower bands uniting into continuous beds. This con- 

 trasts well with the overlying series, and has a thickness of 42 feet. 

 Exogyra hruntrutana is scattered throughout this and the next. 



No. 13 is a strong band of cement-stone, with abundance of 

 Thracia tenera and some other ill-recognizable fossils, 2 feet. 



No. 14. Another mass of indurated sandy marl, 30 feet in thick- 

 ness, and containing Rliynchonella jjortlandica. 



No. 15. Another, but thicker, cement-stone, nearly 5 feet. This 

 is probably nowhere exposed in Portland. 



No. 16 may be taken as the rest of the sand, the line of separation 

 between it and the Kimmeridge Clay being more or less arbitrary. 

 There are indurated bands, and more marly beds, which throw out 

 the water. In the lowest of the subdivisions thus made, but nearly 

 40 feet from the base of the whole, Lingula ovalis is rather abundant, 

 which seems to me a significant fact. The total amount of this No. 16 

 is about 126 feet. 



Adding the thicknesses of these several beds, we arrive at 244 feet, 

 which is demonstrated to be not far removed from the true thick- 

 ness of the Portland Sand here. If this seems extravagant it 

 may easily be checked. The height of the cliff is about 500 feet. 

 It is capped by 50 feet of Portland Stone ; and my former estimate 

 of the Kimmeridge Clay, taken from the same limiting lino to the 

 base of the cliff, was 173 feet, which Avould leave 277 feet for the 

 Portland Sand if all were horizontal. 



The description of this district would end here but for the fact of 

 the French geologists introducing the terms Middle and Lower Port- 

 landian, and asserting the latter to be absent from England. I have 

 therefore reexamined the Kimmeridge Clay here in order to cor- 

 relate it with the strata of Boulogne (see fig. 1). The measure- 

 ments of the upper Kimmeridge have been checked, and it has been 

 ascertained that the clays of Kimmeridge Bay are really lower than 

 any to the east by their fauna and succession, and have a total thick- 

 ness as actually seen of about 183 feet. They reach the anticlinal 

 at the western end of the bay, and then dip rapidly down and towards 



