part 1] GAULT AlS^D LOWER GREENSAND NEAR LEIGHTON. 65 



be proved that their ferruginous condition was attained before they 

 were covered by the newer sediments. In several instances it may 

 be surmised with probability that the beds were originally giau- 

 conitic, and that the decay of this constituent was brought about 

 not long after their deposition, by some physical change by which 

 the material was exposed to weathering agencies. It is likely 

 enough that slight oscillations accompanied the general downward 

 movement of the period, and that the shoals and sand-banks of the 

 shallow sea may at times have been raised above low tide, if not 

 above all tides. ^ The iron-pan and breccia above the Shenley shoal 

 suggest conditions of the lateritic type, possibly in operation above 

 or between tide-marks, or in very shallow water. I am not qualified 

 to speculate upon the chemical history of the processes ; but an 

 examination of the structures in the held shows clearly enough 

 that the iron-pan has been produced, not directly as a precipitate 

 or sediment, but by the replacement of some constituents of 

 an existing deposit and the cementation of the rest. All the evi- 

 dence goes to show that it was in practically its present condition 

 before the clays of the Grault were laid down, and that, wherever 

 it remained unbroken, it protected the beds beneath it from the 

 scour of the currents which sw^ept the strait until well into Gault 

 times. 



During this heavy scour no sediment could lodge on the bare 

 smooth ironclad surface of the reef ; consequently we find that the 

 glauconitic sands wedge out on its flanks (fig. 12, p. 22) and that 

 the gritty glauconitic loams, etc., with their phosphatic nodules, 

 are preserved only in the hollows and gullies by which the edge of 

 the reef is broken, and in the tract on the south where the water 

 was deeper. Fortunately, the lenticles of limestone on the top of 

 the reef afford us a glimpse of its fauna at the beginning of these 

 events, a fauna consisting for the greater part of forms adapted 

 for rocky ground, such as the anchoring brachiopoda and the usual 

 moUusca, echinoderms, and crustaceans of the ' reef-facies ' ; but, 

 after this, the sequence is locally broken until the waters above 

 the shoal became deep enough and still enough to allow a cla}^- 

 mantle to be spread over it. 



The Grault Clays (4 & 5). — Having supposed that the Gault 

 of the district, as a whole, had been already fairly well elucidated 

 by the work of previous observers, particularly of Jukes-Browne & 

 Hill, 2 I have not, until recently, devoted much attention to any 

 except its lower portion. However, in consequence of the statement 



^ Many features of these Basement beds have their analogues in the 

 ferruginous Dogger at the base of the Oolites in Yorkshire, which shows the 

 same local variability on a bigger scale, the same association and changes 

 between ferruginous and phosphatic -nodular beds, the same ' Tourtia '-like 

 conditions in its relation to the underlying strata, and similar palaeontological 

 evidence for ' condensed ' deposition. The patchy local concentration of iron- 

 ore in the Dogger, though on a much larger scale, is also comparable. 



2 ' The Gault & Upper Greensand ' Mem. Geol. Surv. 1900, chaps, xix & xx. 



. J. G. S. No. 309. F 



