23: 



NATURE 



{Jan. II, 1877 



and at Studland, forms the whole thickness of the cliffs 

 between Poole Harbour and Bournemouth, We thus 

 have a magnificent section four miles long and 100 feet 

 in height. Branksea Island is also formed of this series. 

 Their entire thickness cannot yet be accurately stated, 

 but may be put down at some 300 feet. They are cha- 

 racterised by the fact that the clays contained in them are 

 usually brick- earth. 



The next series above is a marine series and is some 

 400 or 500 feet thick. The base beds are dark sands and 

 clays, succeeded by pebble beds and sands, then more 

 sandy clays with pebbles, and ending with a thick deposit 

 of white sands. This marine portion of the series occupies 

 the cliffs between Boscombe and High Cliff. 



Plain as this order of deposition appears, we have col- 

 lateral proof that this interpretation is right, for at Alum 

 Bay there is a complete section of the whole of these 

 beds although somewhat thinned out, upheaved verti- 

 cally, that is, turned completely on end, so that we 

 can examine them in detail in the space of a few 

 hundred yards, like passing in review volumes on a 

 book-shelf. We see in succession the lower pipe-clays, 

 the briUiant sands, the darker clays, sands, pebble- 

 beds, one after the other, so tilted up and so placed that 

 those who know nothing of the depression and elevation 

 of areas can with difficulty be brought to believe that 

 they have all been deposited horizontally. 



In giving you the history of the deposition of these 

 beds, I shall have to speak of a sinking area, and before 

 doing so let me remind you that in Lyell's " Geology," a 

 book in everyone's hands, many instances are recorded, 

 of sinking areas in historic times, from our knowledge of 

 which we feel justified in supposing that there were sink- 

 ing areas in geological times. 



The thick pipeclays and quartzose gtits which we find 

 at the bottom of the series can without the shghtest hesi- 

 tation be referred to the result of the wearing away of 

 granite rock, for wherever granite is worn away by water, 

 there we find white clays and similar quartz grits. We 

 need not go further than Cornwall to see still finer clays, 

 which have been produced in quite recent times from 

 granite by the agency of water. The beds of this district 

 included in the Tertiaries, first laid down over the chalk, 

 were those now called London clay (a mai-ine deposit), 

 and when the streams which brought down our Bagshot 

 beds first spread out their deposits, they spread over the 

 London clay, except, perhaps, in those places where they 

 first cut away the London clay, so that some of these 

 Bagshots were possibly laid down on the chalk. The 

 water in this case came from the west, and as here we 

 are nearer the hills, which were the source of the clay, 

 we find the grits coarser and the clays thicker. 



At Studland the grits are not so coarse, and at Alum 

 Bay, a long way east, the sands are very fine, so that 

 anyone knowing the district could tell which of these 

 specimens came from either place. 



Each clay-patch represents a small lake, first scooped 

 by the running water out of the beds just previously 

 deposited, and then filled in by sediment. The mode 

 of action is this : — The weather disintegrates the 

 exposed surfaces of the distant granite rocks, and the 

 loosened particles are carried by rain into streamlets, 

 which convey them on to the river. The river, 

 tearing and tumbling along, grinds the rocks which 

 have fallen into its bed into round boulders, until in flood 

 times the water is white with finely-divided granite. This 

 grit being hurried along by the rush, is spread far and 

 wide over the valley whenever the stream bursts its banks, 

 which mountain torrents very often do, while the finer 

 particles are still held and carried on until a lake or pool 

 is met with, where the speed is checked ; these fine par- 

 ticles are then dropped, and the water becomes quite clear. 

 This deposition of fine clay goes on for ages, until the 

 lake becomes filled up, the water gets diverted into 



another channel, and what was a lake becomes dry land ; 

 the river at the next flood spreads over the valley, covers 

 in common with the surrounding ground what was the 

 lake again and again with thick grits. Such is the origin 

 of the lai-ge basins containing the clays which serve now 

 to make your pipes and your crockery. You have only 

 to recognise that the valley in which this takes place was 

 slowly sinking, and there is no limit to the thickness of 

 sands and clays which might be thrown down on any one 

 spot, and in this way can be explained the sudden 

 changes from grit to clay, which would else be a puzzle 

 to us. The size of these old lakes is very well seen now 

 wherever a clay basin has been quarried away, for the 

 clay is quarried away for use whilst the sand is left. 

 Some of them are represented by the beautiful blue pools 

 I told you about, and are seen, thereforcj to have been 

 about one-quarter to one-third of a mile round, whilst 

 their depths have varied from 30 to 6q feet. Mr. 

 Lawrence Pike informs me that other clay basins are of 

 larger extent, being | of a mile in diameter. Their 

 greatest length is in the direction of the valley. These 

 clays extend under the surface, eastward, for they are 

 worked at Branksea under the sea-level, at Parkstone, 

 and near Bourne. At Alum Bay they are tilted up, 

 and are full of beautiful fossil leaves. 



This next series of beds above, which I have told you 

 are of a different character, mark a great change in the 

 conditions of the land. The clay patches are of smaller 

 extent, being the filling in of mere ponds or puddles, 

 which acted on a smaller scale, as the lakes of which we 

 have just spoken. The change indicated by these beds 

 is one from the valley in which the previous contained beds 

 were deposited, to a broad low-lying tract in proximity 

 to the £ea. We infer that we can trace how this tract 

 became gradually lowered and lowered down to the sea- 

 level. 



The belief in the gradual lowering of the land in this 

 area is borne out by the fact that in the cliffs near Poole, 

 which are slightly lower in position than those farther 

 east, we get only leaves of evergreens and forest trees, 

 whilst as we work our way east so as to meet with beds 

 on a higher level or, which is the same thing, of more 

 recent age, we get a mixture of ferns and other plants, 

 which require much moisture, whilst farther east still we 

 get assemblages of plants that could only have lived in 

 absolute swamps. 



Low as the land appears to have become we have no 

 evidence whatever, throughout the whole thickness of this 

 part of the series, amounting to 300 feet at least, with an 

 exception which I will tell you about directly, that it was 

 low enough to be inundated by the sea, as the few shells 

 that have been found are of fresh-water kinds. The 

 exception alluded to is the occurrence of logs of wood 

 bored by the ship-worm or teredo. All the ship-worms 

 generally known to us live only in salt water, and are so 

 delicately organised that the slightest mixture of fresh 

 water instantly kills them. This isolated fact for some 

 time presented a grave difficulty, but happening to readj 

 Mr. Gwyn Jeffreys' interesting account of the habits oif 

 this creature, I not only found that he relates the occur [ 

 rence of similarly bored wood 300 miles up the Rive j 

 Gambia, but distinctly states that there is a specie: 

 which lives in fresh water. Therefore this suppose(| 

 marine indication may be on his authority removed, ancl| 

 supposing this theory should be verified and universall 1 

 accepted, we may safely infer that these middle beds arj 

 of fresh-water origin. 



We now come to the third series of beds. A still cor| 

 tinued sinking of the area brought this swampy conditio j 

 so low that the sea was no longer kept out, but, burstin 

 through, formed great salt-water lagoons teeming wit 

 life ; for we suddenly find crowds of marine forms inj 

 bedded in what was formerly black mud, such as v| 

 might find now in the existing Poole Harbour here. 



