412 H. A. Baker — Loose Arenaceous Sedhnents. 



deposits of the geological column are the results of transport by and 

 deposition from comparatively slow-moving currents. Deposits 

 of torrential character are exceptional. Consequently, we should 

 expect to find far-travelled natural sediments, which have been 

 subjected to long-continued aqueous action before deposition, 

 showing in a marked degree the results of selective treatment, 

 and sediments deposited after a short career under transport showing 

 very little evidence of sorting. The material composing arenaceous 

 sediments deposited under marine conditions must, in general, 

 have had an earlier history, during which it formed part of fluviatile 

 or estuarine deposits, and in view of the greater vicissitudes 

 experienced should show the results of selective action in a high 

 degree. On the other hand, a deposit such as a fluviatile loam or 

 brickearth, deposited after only a short career under transport, 

 and not having been subjected to any special type of selective 

 action, should show a marked absence of sorting. The expectation 

 is verified. Fig. 3 shows, for purposes of comparison, the typical 

 elutriation-curves yielded by fluviatile, estuarine, and marine 

 sediments. In the case of the fluviatile deposit, its immaturity, 

 from the point of view of its modification under aqueous action, is 

 shown by the curve by the marked absence of a " hump " and by 

 the very low grading factor. The marine sediment yields a curve 

 with a pronounced " hump " and gives a much higher grading 

 factor. The estuarine sediment is of intermediate type. Many more 

 examples might be given to show the soundness of this generalization, 

 but they are scarcely necessary at this point. 



Mechanical Constitution of the Thanet Beds oe the Southern 

 Side of the London Basin. 



The following summary of the leading features of the mechanical 

 constitution exhibited by the Thanet Beds is based upon the results 

 of a very large number of analyses of samples from a widespread area. 



Everywhere along the southern side of the London Basin the 

 Thanet Beds are always progressively coarser from the base upwards. 

 A sample from the upper part of the Thanet Beds in any locality 

 will always yield a higher Equivalent Grade factor than will another 

 taken from a lower level. Samples taken from similar horizons in 

 different localities do not, however (unless the localities are only a 

 mile or two apart), yield corresponding figures. The Thanet Beds 

 become finer in grain as we proceed both eastward and westward 

 from the type district of West Kent. The Thanet Beds of East 

 Kent, as shown by the sections at Pegwell Bay and Heme Bay, 

 are the most fine-grained representatives of these deposits in the 

 southern part of the London Basin (see Fig. 4). It is only, however, 

 in the upper part of the beds that the transition from coarser to 

 finer material, eastward and westward of West Kent, is steadily 

 progressive. The lower part shows greater variation. The lower 

 portion of the beds in the West Kent type area shows finer sediments 



