378 



REV. A. IRVING ON THE PHYSICAL HISTORY OE 



bedding-planes, of which "oblique lamination" is a minor feature. 

 That this is not a necessary indication of Lower-Bagshot horizons 

 is seen from the simple fact that it is very commonly observed in sec- 

 tions of more coarsely arenaceous beds of the Middle division, and 

 serves as one general character wherewith to link the two together, 

 as on the whole representing a continuance of similar physical con- 

 ditions, while it helps to differentiate them from the Upper Sands, 

 in which the bedding is for the most part very indistinct. 



3. Pipe-clay. — This need not owe its origin in these beds directly 

 to the well-known process of kaolinization of felspathic minerals ; 

 it seems to me more likely that it owes its existence in them to the 

 decomposition of the chalk and the leaching out of the iron by the 

 action of peaty acids. Recent experimental study of this question 

 has tended to confirm this view, so that I am inclined to regard the 

 prevalence of pipe-clay as a possible indication of contemporaneous 

 subaerial denudation of the Chalk, and of its elevation, therefore, 

 above the sea, within the catchment-basin whose waters were drained 

 into this Bagshot area. But this applies to the Middle as well as 

 to the Lower group ; in fact, there are greater accumulations of 

 this mineral in the former than in the latter *. This being so, we 

 cannot admit the presence of pipe-clay seams as conclusive evidence 

 of a Lower-Bagshot horizon f . Further, thin layers of colourless 

 clay are occasionally met with in sections of the Upper group in the 

 interior of the district, as well as in some of the more sandy portions 

 of some of the plateau-gravels.. 



4. Derived Materials. — The possible occurrence of such from the 

 older Tertiaries in the Middle group at BrookwoodJ has been 

 already indicated, and Prof. Rupert Jones has suggested the appear- 

 ance of derived masses of clay in beds of the same group §. 



5. Irony Concretions. — These are at least as abundant in the 

 beds of the Middle division as in those of the Lower. They occur 

 in the uppermost bed of the former frequently enough. They often 

 retain impressions of vegetable structures, or they form rough hollow 

 tubes, just as recent bog-iron-ore often does. Some caution is 

 needed in interpreting an apparent annular structure as a " sign of 

 wood," since, as Roth has pointed out ||, this structure is often due 

 to the rolling in a dry season by the wind, and subsequent burial, 

 of partly dried and curled-up portions of the carbonaceous and 

 ferruginous mud which is found at the bottom of stagnant pools of 

 water. I have in my possession specimens of ferruginous concretions 

 from a high horizon in the Middle division, which retain impressions 

 of vegetable structure, suggestive of such marsh-loving monocotyle- 

 don ous plants as belong to the orders Cyperacece, Typliacece, &c, 



* Cf. the Brookwood Well-Section (Geol. Mag. dec. iii. vol. iii. p. 355) ; the 

 Well-section at Wellington College (Quart. Journ. Geol. Soc. vol. xli. p. 494) ; 

 also the instance mentioned in the Survey Memoir on the London Basin (vol. iv. 

 p. 833) near Worplesdon. 



t See Quart. Journ. Geol. Soc. vol. xlii. p. 411. 



\ Geol. Mag. loc. cit. 



§ See Proc. Geol. Assoc. vol. vi. p. 433 (footnote). 

 || 'Allegemeine una chemische Geologie,' p. 597. 



