OCR Text |
Show 286 EOCENE PERIOD. [Ch. xxr. found ncar Newhaven, and at other points, as will be seen by the map. These are now wasting away, and will in time disappear, as the sea is constantly encroaching and undermining the subjacent chalk. The secondary rocks, depicted on the map, may be divided into five groups:- 1. Chalk and Upper green-sand.-This group is the uppermost of the series ; it includes the white chalk with and without :flints, and an inferior deposit called, provin~ cially, 'Firestone,' and by English geologists the' Upper green-sand.' It sometimes consists of loose siliceous sand, containing grains of silicate of iron, but often of firm beds of sandstone and chert. 2. Blue clay or calcareous marl, called provincially Gault. 3. Lowe·r green-sand, a very complex group consisting of grey, yellowish, and greenish sands, ferruginous sand and sandstone, clay, chert, and siliceous limestone. 4. Weald clay, composed for the most part of clay without intermixture of calcareous matter, but sometimes including thin beds of sand and shelly limestone. 5. Iiastings sands, composed chiefly of sand, sandstone, clay, and calcareous grit, passing into limestone*. The first three formations above enumerated are of marine origin, the last two, Nos. 4 and 5, contain almost exclusively the remains of fresh-water and amphibious animals. But it is not our intention at present to enlarge upon the organic remains of these formations, as we have merely adverted to the rocks in order that we may describe the changes of position which they have undergone, and the denudation to which they have been exposed since the commencement of the Eocene period,-mutations which, if our theory be well founded, belong strictly to the history of tertiary phenomena. By a glance at the map, the reader may trace at once the "' Fur nn account of these strata in the south-east of Englancl, sec Mante!Ps Geolog-y of Sn~sex, and Dr. Fitton's Geology of Hastings, where the memoh·s of all the wrih~1·s on this part of England are rL'f~rr~u to, Ch. XXI.] SECTION OF WEALD VALLEY. 287 superficial area occupied by each of the five formations above mentioned. On the west will be seen a large expanse of chalk, from which two branches are sent off; one through the hills of Surrey and Kent to Dover, forming the ridge called the North Dow~s, .the other through Sussex to the sea at Beachy Head, const1tutmg the South Downs. The space comprised between the North and South Downs, or 'the Valley of the Weald,' consists of the formations Nos. 2, 3, 4, 5, of the above table. I~ will be observed ~hat the chalk terminates abruptly, and with a well-defined lme towards the country occupied by those older strata. Within that Jine is a narrow band coloured blue formed by the gault, and within this again is the Lower greet~ sand, next the Weald clay, and then, in the centre of the district, a ridge formed by the Hastings sands. Section of the Valley of the Weald.-It has been ascertained by careful investigation, that if a line be drawn from any part of the North to the South Downs, which shall pass through the central group, No. 5, the beds will be found arranO'ed in the order described in the annexed section (No. 63, p. 2S8). We refer the reader at present to the dark lines of the section, as the fai.nter lines represent portions of rock supposed to have been earned away by denudation. At each end of the diagram the tertiary strata a are exhibited reposing on the chalk. In the centre are seen the HastinO's sands (No.5), forming an anticlinal axis, on each side of whi~h the other formations are arranged with an opposite dip. It has b~en necessary however, in order to give a clear view of the d1ffer~nt formations, to exaggerate the proportional heiO'ht of ~a.ch 111 comparison to its horizontal extent, and we haven subJOmed a true scale in another diagram (No. 61) in order to correct the erroneous impression which might otherwise be made on the reader's mind. In this section the distance betw~en the North and South Downs is represented to exceed ~0 miles; for we suppose the valley of the Weald to be here lntersecte~ in its longest diameter, in the direction of a line between Lewes and Maidstone. |