500 A. Tylor — Formation of Deltas. 



Two principal beds illustrate the richness of these water-bearing alluvial deposits. 

 The first, at forty-four to fifty-five metres, is so charged with gas, that at some points 

 the expansion of the carburetted hydrogen gas projects the earthy matter (from the 

 bore-hole) to a height of twelve metres. If a light is presented at the orifice of the 

 boring from which the gas escapes, an explosion follows, and the gas continues to 

 burn. The second water-bearing bed lies at an even depth of sixty metres ; it is not 

 so charged with gas as the first, and the water flows from it constantly and with great 

 regularity ; it is found everywhere, but occasionally the pressure of the water varies. 



In 1850, these borings yielded 1,029 litres of water per minute. 



Degousee et Laurent. 



Plate XL, Fig. 2. — Section along a line of eight miles from Folkestone to Dover, 

 passing by the Holywell Spring B' B", thence to the watershed W on the west of . 

 the Sugarloaf Hill (a marked feature of the escarpment of the South Downs), thence 

 by Alkham to the village of River and to Dover. See V V" V". Sugarloaf Hill 

 projects a little beyond the line of the main escarpment of the Chalk, but not so 

 much as Beechborough Hill near Cheriton, which has almost the appearance of a 

 tumulus. There is a line of springs and brooks along the whole escarpment; but 

 the level of the springs gradually falls as it approaches the sea, or one of the Wealden 

 Rivers. The level of the escape water is reduced thus, from 180 feet at Cherry 

 Gardens to 140 feet at Holywell, about half a mile, at Lydden Spout, three miles 

 further immediately on the coast the water escapes not far from high-water mark. 

 The Springs above River, five miles from Holywell, are at a level of 112 feet above 

 the sea (Fig. 2). The supposed underground water level is shown in Fig. 2 by a 

 line A' A" A'". This varies very much from year to year. In wet seasons the water 

 level rises rapidly. When it changes as much as 20 feet, the springs overflow in a 

 large bourne at Drillincourt (Fig, 2), making a temporary river. 



Plate XL, Fig. 3 represents the alternations of coombs and headlands along the 

 valley and hill separating Folkestone and Dover, or along the escarpment of the Downs. 

 This is only a diagram and it does not attempt to describe any local features. It is 

 drawn to show the manner in which all headlands and coombs alternate in every 

 valley or escarpment, and along all lines of coast section, more or less symmetrically. 

 The Springs S' S" S'" rise near the "Watershed W W" in several points, and 

 then collect in one channel, flowing into the cross stream at the bottom of the 

 Valley V V" V", always joining it in a single stream at an acute angle. The upper 

 terminations are often trifid. The different distances from the watersheds, the 

 flexures in the impervious and pervious beds aifect the quantity discharged fi"om each 

 side stream S' S", and introduce local changes in their relative positions and magni- 

 tudes aflFecting the sizes and distances of the headlands. In a map of Crete, by 

 Capt. Spratt, the distances of the side streams from each other are surprisingly 

 regular. Fig. 3 shows the form of denudation in binomial curves (page 6), which 

 govern the general shape of all headlands and coombs, hills and valleys ; the hard or 

 soft beds or clays also afi'ect the stability of all strata, and produce deviations from the 

 true binomial curve of denudation which are not attempted to be represented. The 

 line B' B" B'" in Fig. 2 would represent the transverse section of the headlands and 

 coombs in Fig. 3, The remains of watercourses of the Pluvial Period are less visible 

 on chalk escarpments than on any others, but in chalk valleys they can be traced. 



Plate XL, Fig. 4 is a drawing of one of the infinite numbers of binomial curves of 

 different powers and lengths that may be described. The base is divided into equal 

 portions or abscissae, and the ordiuates are to scale, their lengths corresponding to 

 the co-efficients of (a+by. This form of curve was applied by Quetelet to repre- 

 sent the variations of lengths and numbers to illustrate certain statistical results; 

 but the writer believes he was the first to indicate that actual accumulations as well 

 as removals and elevations of materials forming the earth's surface, actually assumed 

 a form which could be represented by binomial curves. He has measured beaches, 

 flexures, as well as surfaces of valleys and hills, denuded by water, and found them 

 coincide nearly with binomial curves. The binomial might be called the geological or 

 l^hysiographical curve, or the curve of denudation, or deposit, so marked is it in 

 nature. It is essentially the heap curve. Everything heaped up, whether water in 

 waves, or solids, like hay when heaped up, follow this curve. One of the principal 

 characteristics of all binomial curves is that the upper part is convex, and the lower 

 concave. The curve of a stream commencing at a watershed begins at 0, following a 

 convex course, and gradually becomes steeper towards the middle portion, then 



