28 Notices of Memoirs — W. Whitaker's Address. 



northern part of our county some parts of the large sheet of brick- 

 earth are sandy, and therefore fall into the Mixed division. Here, 

 again, I have drawn on Mr. Woodward's knowledge ; but neverthe- 

 less I can only look on part of the Norwich map as an approxima- 

 tion or a compromise. 



When we come to the later, or Post-Glacial Drifts, as before, the 

 gravels and sands range themselves in the permeable beds (of course 

 I mean only when not underlain by impermeable or mixed beds) ; 

 but the brick-earth is rarely impermeable, being generally of a sandy 

 nature, and therefore classed as Mixed. 



On the broad Chalk tracts of some of our more southern counties 

 that rock is to a large extent hidden, over the higher grounds, by a 

 very irregular mass of brick-earth, usually with a more clayey layer, 

 known as the "clay-with-flints," at the base. Now this brick-earth 

 is also sandy, and though the underlying clay would seem at first 

 sight to be fairly waterproof, yet the fact that it has been formed 

 simply by the gradual dissolving away of the Chalk, by the infiltra- 

 tion of carbonated water, is enough to show that water has sunk 

 through to the Chalk in large quantity — the clayey matter being 

 indeed simply a filtrate, the solid residue left after very long con- 

 tinued action, I have therefore classed these loamy and clayey tracts 

 as Mixed ; but they need not trouble us here, as they do not occur in 

 Norfolk. 



Alluvium. — It might be expected that the deposits of our marshes 

 and river-flats would be impermeable, from their generally clayey 

 nature ; but I have thought it advisable as a rule to class them in the 

 Mixed group. The reasons for this are that some alluvium is of a 

 sandy nature, that some is very thin, and that the water-courses in 

 many places cut through the alluvium to the gravel and chalk beneath ; 

 so that on the whole it is best to class it amongst the doubtful beds. 



With regard to the four divisions adopted in these maps the first 

 two. Bare Chalk and Chalk covered by permeable beds, naturally 

 group themselves together, as also do the latter two, the areas taken 

 up by mixed beds and by impermeable beds. It should be noted, 

 however, that there are large tracts of the last division in which the 

 natural drainage is outward, towards the chalk, in consequence of 

 ■which much of the rain falling on such tracts of impermeable beds 

 flows across them to the Chalk, or to permeable beds over the Chalk, 

 and then in many parts wholly sinks into the ground, and in others 

 partially sinks ; so that these areas contribute to the supply of water 

 in the Chalk. In some of the newer maps exhibited I have divided the 

 area of the impermeable beds into two, distinguishing the part that 

 drains away from the Chalk from that where the drainage flows to 

 the Chalk. 



Of course to make use of these maps it is needful to measure the 

 various areas, and a very hasty look at the maps will show you that 

 this will be a very troublesome task. It has been done however 

 on the older maps exhibited, by engineers with the proper appliances 

 for such work ; but I have not ventured to attempt such measure- 

 ments on the new maps. 



