GO 



THE GEOLOGIST. 



genera Cytherea, Cyprina, Cardium, Pectunculus, Natica, and Lamna 

 teeth) ; but there are also a few of the estuarine shells of the Woolwich 

 beds, and in abundance Cyrena, Cerithium, and Melania. 



This bed (or rather the set of sands, etc., that are usually classed with 

 it) does not occur for some distance to the east, owing to the extent of the 

 denudation of the Tertiaries ; coming on again, however, near Sittingbourne 

 with the same general characters and a little thicker. It has been observed 

 in the cutting north of that town, on the Sheerness branch-line. Near 

 Boughton and Canterbury this sand is again shown, and it contains flint- 

 pebbles and ironstone. At the mouth of the tunnel, on the Whitstahle 

 Railway, near the second place it may be well seen, and the thick pebble- 

 bed at the base, cemented into a hard mass by the ironstone, is remarkable. 

 At Shottenden Hill there is a great thickness of the flint-pebbles. 



The fine cliff-section between the Heculvers and Heme Bay shows this 

 sand-bed very clearly. It has here an average thickness of 20 feet ; is 

 crowded with fossils ; contains large blocks of hard tabular sandstone, often 

 almost made up of fossils or casts of fossils, and smaller blocks of fossil- 

 bearing iron-sandstone ; and has a well-marked pebble-bed, from a few 

 inches to more than a foot thick at the bottom. The fineness and loose- 

 ness of the sand of this division of the Tertiary series is here well shown : 

 on windy days many tons' weight of this sand are blown away from the 

 face of the cliff, the wind indeed being very powerful in wearing away 

 these cliffs. The fossils are of a London Clay type ; but many of them 

 occur also in the beds below r . Sharks' teeth and vertebrae are plentiful, 

 and amongst the shells the bivalve genera Cyprina, Cytherea, and Car- 

 dium, and the univalves iNatica and Aporhais. 



In all these East Kent sections, it is to be noted that the London Clay 

 is sharply divided from, and nowhere passes into the sands below, and that 

 it nearly always has a few pebbles, green grains, and sharks' teeth at the 

 bottom, which is rather sandy, and not unlike the undoubted basement-bed 

 in the western part of the London basin, and this may perhaps be the real 

 basement-bed. The underlying sands are more like the beds below them 

 in mineral structure ; but more allied to the London Clay by fossils. 



4. The London Clay is not only of much greater thickness, but also of 

 more uniform. mineral character than the lower formations. It consists al- 

 most throughout its whole thickness of stiff dark-bluish-grey clay, weather- 

 ing brown, with now and then a bed of nodular masses of clayey limestone 

 (known as " septaria," from having septa or divisions of carbonate of lime), 

 from which Roman cement is made. The lower part is generally roughly 

 laminated, and abounds in transparent crystals of selenite (sulphate of 

 lime). The upper part, which however rarely occurs, having been for the 

 most part denuded, is also somewhat laminated and sandy, and generally 

 passes up into the overlying Bagshot Sand (present only in the Isle of 

 Sheppey). 



. The full thickness of the London Clay in the most western part of Kent 

 is calculated to have been about 450 feet; so great a thickness occurs no- 

 where in West Kent, however, the highest beds of the formation having 

 been denuded. Shooter's Hill, indeed, is the only place where the greater 

 part of the London Clay has been left. In the Isle of Sheppey the thick- 

 ness is calculated at 480 feet, the greatest known. As might be expected 

 from its greater thickness, the London Clay occurs at the surface over a 

 wider belt of country than the thinner underlying formations. 



The fossils, iu some places abundant, but often " conspicuous by their 

 absence,'' show that the clay is a sea-deposit. Moreover, as the many 

 species occur for the most part from top to bottom of the formation, and as 



