Vol. 50.] ME. B. THOMPSON ON LANDSCAPE MAEBLE. 407 



vegetation. 1 Sometimes the water would be chiefly derived from 

 springs within the watershed, and would contain very little sediment, 

 but being highly charged with bicarbonate of lime (as all limestone 

 springs are, particularly if derived from water firstly percolating 

 through decaying vegetable matter) it would deposit some of the 

 carbonate of lime on reaching the shallow and comparatively 

 stagnant water of the estuary, chiefly through loss of carbonic acid 

 consequent on higher temperature. 



During a rainfall over the watershed the rivers would receive 

 surface-drainage, in addition to the clear supply from springs. This 

 would contain a considerable quantity of organic matter washed off 

 the surface, and particularly the already much decomposed vegetable 

 matter of bogs and natural ditches. 



Thus there would be frequent alternations of nearly pure 

 inorganic matter and mixed sediment, sometimes the inorganic 

 predominating and sometimes the organic ; thus too the layers 

 would vary in constitution and thickness, forming the lower striated 

 portion of the stone. At these freshets also, no doubt, the greater 

 part of the argillaceous matter and grains of quartz, etc., would be 

 introduced. 



Now and again there would be a heavy rainfall and flooding of 

 the low-lying fenland, and then the streams would not only bring 

 much more sediment and organic matter than usual, so as to produce 

 thicker layers of dark matter, but they would by virtue of their 

 greater velocity and volume tend to re-arrange the material recently 

 deposited in the direction of their course in the estuary, entirely 

 sweeping the bed of all recent sediment along certain lines, and for 

 a certain distance, and re-depositing this matter farther out to sea, 

 or in lenticular masses at its side (16). For, where the rapid water 

 of the river touched the stagnant water of the estuary, strong 

 eddies or miniature whirlpools would be produced, into which 

 would be swept the matter (now more than usually organic) which 

 would otherwise have been spread in a thinner layer over a larger 

 area. These eddies thus would form traps for sediment, and, 

 according to the depth, sedimentation would proceed quietly below, 

 and lenticular masses would be formed (16). Outside the area of 

 the eddies the deposit would go on much as usual, the eddies them- 

 selves acting as screens for a time. Where the water was shallow 

 enough the eddies would also disturb the sediment around, and 

 so obliterate all traces of the original stratification, producing a 

 mixed layer not to be distinctly recognized as Cotham Stone (17). 

 Or possibly, in some cases, they would produce depressions which 



1 ' From the frequency of such delicate creatures as insects in the Landscape- 

 stone, and in another band of limestone only a few feet higher, some of which 

 are said to be beautifully preserved, and could not have been long subject to 

 the action of the waves, it is supposed by Mr. Brodie that this part of the Lias 

 may have been formed in an estuary, which received the waters of some 

 neighbouring coasts, and which brought down the remains of insects and 

 plants.'— Herbert Goss, quoted by Dr. H. Woodward in his paper ' On a New 

 Lias Insect,' Geol. Mag. 1892, p. 195. 



