W. TVHITAKER ON SOME BORINGS IN KENT. 41 



often rest at once on Trias, so a like thing may occur in the London 

 Basin ; and, even if the bottom-rock at liichmond should turn out 

 to be Old Eed, it does not follow that the same age must be as- 

 signed to the beds beneath the Gault in the other borings. Old Red 

 moreover is not the only alternative in the Eichmond case, Carbo- 

 niferous rocks being often stained red. 



From this to Chatham (over 19 miles E.S.E,) we have no deep 

 boring passing through the Chalk ; but the whole of this evidence 

 shows that the ridge, or one should say rather the plain, of Primary 

 rocks under the Thames Yalley was under water long before Cre- 

 taceous times, being capped, though sometimes very thinly, by older 

 Secondary beds, varying in age from Oxford Clay perhaps to Trias, 

 but Lias not yet having been found, nor any member of the Upper 

 Jurassic. 



From London northwards we have (besides the Kentish-Town 

 section) two deep borings : — firstly that at Cheshunt (Turnford)* 

 about 13 miles northward of Kentish Town, in which the Gault is at 

 once succeeded by Devonian shale, and then that at Ware (Broad 

 Mead) t about 6| miles further north, where Upper Silurian beds 

 occur at a higher level than any of the other old rocks in our other 

 deep borings, and are separated from the Gault by only 1| feet of 

 Lower Greensand. However, as we have no description of this thin 

 bed, I am somewhat inclined to regard it rather as the base of the 

 Gault. 



Northward from Crossness the Loughton well (of which a de- 

 scription wiU shortly be published by the Essex Field Club) gives 

 little information, having been carried only to the base of the Gault ; 

 and the older Saifron-AValden well gives none, the Chalk Marl, the 

 Gault, and underlying beds having been lumped together, in the 

 accounts that have been preserved, as Chalk Marl, with the 

 impossible thickness of over 720 feet:}:. 



From Chatham north-eastward we reach Harwich, at a distance 

 of 48 miles ; and here alone have we met with any marked irre- 

 gularity in the Gault, which formation is but 61 feet thick, and is 

 underlain by Lower Carboniferous rock. Further north the Combs 

 boring (Stowmarket) § and the ^N'orwich weU || have not bottomed 

 the Gault, and so give us no information as to the older beds. 



I would remark, by way of caution, that, from the proved 

 occurrence of Jurassic clays in some of our borings, the classification 

 of a clay as Gault may be hardly safe, where uncorroborated either 

 by fossil-evidence, as I believe is the case at Ware and at Harwich, 

 or by the occurrence of underlying IN'eocomian beds. We have every 

 reason to expect the constancy of the Gault ; but we should not let 

 this hinder us from seeing the possibility, small though it be, of the 

 absence of that formation. 



* See Trans. Herts Nat. Hist. Soc. vol. iii. pt. v. p. 176 (1885). 

 t Ihid. p. 179. 



X See W. H. Penning in the ' Geological Survey Memoir,' on Sheet 47, p. 79 

 (1878). Also Proc. Norwich Geol. Soc. pt. 1, p. 28 (1878). 



§ ' Geological Survey Memoir,' on Sheet 50 S.W. pp. 19, 20 (1881). 



II Proc. Norwich Geol. Soc. pt. 8, pp. 250, 251 (1884). 



