26 Notices of Memoirs — W. Whitalcer's Address. 



I have found only two certainties, that is, only two colours on our 

 Geological Survey Maps that could at once be followed. These two 

 are the outcrops of the bare Chalk (No. 1) and the areas of the 

 London Clay, which latter of course range themselves under No. 4. 

 Moreover, it is not only where London Clay is shown as occurring 

 at the surface that this holds ; but also over the tracts where it 

 occurs beneath other beds, so that, for our purpose, the whole of the 

 Bagshot Sands (mostly permeable though they be) are coloured with 

 impermeable areas as regards the Chalk, and so also will be great 

 part of the Suffolk Crag tract, although that Crag is highly permeable. 



As far as I have yet carried the work, one other colour on the 

 Survey Maps can also be followed, for the Boulder Clay can be 

 included with impermeable beds. However, in some parts in which 

 this work is not yet done, it is possible that there may be some 

 tracts coloured as Boulder Clay over which that bed has been 

 altered, by surface actions, so that some of the clayey matter has 

 been lost and the Chalk dissolved away, the remainder forming 

 a stony loam that is not quite impermeable. Indeed, the fact of 

 such alteration occurring at all, produced as it is mainly by the 

 infiltration of water, is in itself a proof that the Boulder Clay is to 

 some extent permeable, and therefore where thin it may let some 

 amount of water through to more permeable beds underneath. 

 Moreover, on some of the higher ground of West Norfolk, it is not 

 easy to distinguish a thin capping of Boulder Clay from the weathered 

 surface of the Chalk itself, for the Boulder Clay in those parts 

 consists sometimes of little else than Chalk, being mainly a mass of 

 chalk pebbles in a chalky matrix, with a little admixture of sand, 

 but with little clay. It follows therefore that some Boulder Clay 

 cannot fairly be coloured as impermeable ; but must be classed with 

 the mixed beds (No. 3). 



Of course with this Clay, as with the London Claj^ it is not only 

 where it crops out, but also wherever it underlies other beds, which 

 may be highly permeable, that we have to take it into account ; this 

 however, is one of the things that is sometimes not shown on the 

 Survey Map ; but which must be shown from knowledge of the 

 ground, or by inference, from the lie of the beds. 



Having drawn your attention to the more certain parts of the 

 work, it may be well to treat of the various beds in stratigraphical 

 order, premising that in all cases (unless otherwise mentioned) only 

 their area of outcrop is referred to, and not parts where they are 

 covered by Drift, of whatsoever character ; and I will begin at the 

 bottom. 



Thanet Beds. — This comparatively local base of the Tertiary 

 Series is almost wholly a fine sand in Surrey and West Kent, and 

 may then be fairly treated as on the whole permeable. When, how- 

 ever, the map of East Kent is taken in hand, a different condition 

 will have to be dealt with, for then we find a mass of clayey beds in 

 the sands, which may cause the greater part to be impermeable, or 

 at all events not higher in the scale than the mixed beds (No. 3). 

 In Sufiblk the outcrop of this division is too narrow to be shown 



