Mr. Whitaher^s Address to the Geological Section. 465 



certain, indeed, that this should be given as a few feet more, for the 

 22 feet next beneath, which have been described as Gault mixed 

 with Greensand, is probably in part the green clayey glauconitic 

 base of the Chalk Marl. We may fairly add for this another 5 feet 

 (as also in the case of the Combs boring), and may say that, in 

 round numbers, the Chalk reaches a thickness of about 900 feet 

 in the south-eastern part of Suffolk. Toward the northern border 

 of the county it is probably more, as the deep boring at Norwich 

 passes through nearly 1,160 feet of Chalk, and that without be- 

 ginning at the top of the formation. 



Of our recorded Suffolk wells only three reach the base of the 

 Chalk, at Mildenhall, Culford, and Combs ; consequently we have 

 little knowledge of the divisions of the Chalk. These divisions, 

 indeed, are of comparatively late invention, having been evolved 

 since the publication of many of the deep sections that have been 

 referred to. 



If the Upper Chalk at Harwich goes as far down as the flints, 

 then we must allow it to be 690 feet thick, leaving little more than 

 200 for the Middle and Lower Chalk together. At Landguard Fort, 

 from the same point of view, the Upper Chalk would certainly be 

 500 feet thick, and one cannot say how much more. 



At Combs, on the other hand, flints have been recorded as present 

 only in the top 27 feet of the Chalk ; but whilst this may have been 

 owing in part to the boring having passed between fairly scattered 

 nodules, and in part perhaps to insufficient care in observation, at 

 Harwich it is possible that some flints may have been carried down 

 in the process of boring. 



What evidence we have tends to show, however, that the Upper 

 Chalk forms a good deal more than half, and perhaps about two- 

 thirds, of the formation, the Middle and Lower Chalk being rather 

 thin. This agrees with what is found in other parts where the 

 Chalk is thick, extra thickness being chiefly due to the highest 

 division. The glauconitic marly bed at the base seems to be well 

 developed and to be underlain by the Gault clay ; so that we have 

 no good evidence of the occurrence of the Upper Greensand. This 

 division may be thinly represented at Mildenhall, but it is difficult 

 to classify some of the beds passed through in the old boring there. 



As far as the Gaidt is concerned little of course is known; but 

 that little points to this formation being unusually thin, presumably 

 only 73 feet from top to bottom at Culford, and probably not more 

 than between 50 and 60 at and near Harwich. In the north-western 

 part of the neighbouring county of Norfolk it is well known to be 

 still less, the clay thinning out northward along the outcrop, until 

 at last there is nothing but a few feet of Red Chalk between the 

 carstone of the Lower Greensand and the Chalk. The Gault being 

 of much greater thickness around and under other parts of the 

 London Basin, this thinning in Norfolk and Suffolk is noteworthy. 

 The absence of the more inconstant LTpper Greensand is to be ex- 

 pected in most places, and calls for no remark ; it may, however, 

 be noted that geologists are coming to the conclusion that these two 



DECADE IV. VOL. II. — NO. X. 30 



