NEWLY EXPOSED SECTIONS AT READING. 455 



attention, not only to the excellent sections at present exposed, but 

 more particularly — 1st. To the local absence of Oyster-shells (in 

 Section No. 3) within 40 paces of a rich portion of the old Oyster- 

 bed, and in a deposit similar to and continuous with the loamy green 

 sand full of Oysters. This barren margin of the Oyster-bank may 

 have been infested by some material or agent inimical to life, though 

 the borers which perforated the underlying sea -bed of Chalk had 

 existed previously in abundance. 2ndly. The absence of the leaf- 

 bearing clay-beds in the yellow sands of Sections Nos. 3, 2, 1, & 6. 

 The seam of lignitiferous clay, 3 inches thick, and the tough blue 

 shale, nearly 3 feet thick, must terminate within a very few yards 

 west of Section No. 4; whether by the dying-out of lenticular 

 deposits, or by thinning or truncation due to erosion, is not clear, 

 the section being there obscured. We think that the latter con- 

 dition probably exists, as shown by the apparently drifted materials, 

 such as clay-galls and varying seams of clay and lignite fragments, 

 in Sections No. 2, No. 1, & No. 6. 



In this case we regard the yellow sands of Sections Nos. 3, 2, 1, 

 and 6 as having been rearranged in a kind of " Horse-fault," after 

 an earlier deposition of strata, comprehending a westward continua- 

 tion of the grey shales. These were removed by currents eating 

 away a bank, or forming a channel, where the shale had existed, 

 and leaving some remnants of its material, rolled and scattered 

 in the present beds (see plan of the current in an estuary, and 

 formation of shoals at the expense of older beds, PI. XXII. fig. 1). 



Such a circumstance, common enough in both old and recent 

 formations, supplies one more fact in the history of these very in- 

 teresting " Heading Tertiaries ; " and if the actual line of this eroding 

 current can be found by further observation, when Wheeler's Pit 

 is cut back westward in a line with Sections Nos. 4 & 3, and com- 

 pared with the tide- or stream-currents which may be made out of 

 the false-bedded sands by application of Mr. Sorby's formula of 

 " Drift-currents," so much the more will be known of the hydro- 

 graphy of the Early Tertiary Period. 



The hypothetical current, causing the presumed Horse-fault, 

 may have existed, though not with so much force, previously, 

 whilst the Oyster-bed was being formed, and may have interrupted 

 the deposition of Oyster-spat and the growth of Oysters in the area 

 opened by Section No. 3. 



3rdly. The history of the larger clay-galls, above the blue shale, 

 in the yellow sands of Section No. 5. The excavation of the 

 existing valley, and the absence of sections in Castle Hill make it 

 impossible to trace these large clay-galls to any source on the north 

 or north-east. It may be that they were derived from the destruc- 

 tion of part of the smaller and higher of the two leaf-beds seen in 

 place in Section No. 4 — or from a small cliff cut in a thicker portion 

 of the grey shale itself (see fig. 2, showing the supposed destruction 

 of the clay band), and that they lie in sand laid down on a flat sur- 

 face denuded out of this clay-bed. 



We have already suggested that this same grey clay was cut 



