Yol. 52.] EOCENE DEPOSITS OF DORSET. 493 



unable yet to say to what extent the position of these outliers is due 

 to erosion as distinguished from folding. The Rev. 0. Fisher states l 

 that when the large ballast-pit on Bincombe Down was open during 

 the making of the Ridge way cutting, the Tertiary strata were seen 

 to be vertical. Sir Joseph Prestwich* and Mr. Strahan, 3 however, 

 in their sections, draw the Eocene base as markedly unconformable 

 on the upturned Chalk. All that can now be clearly made out is 

 that in certain parts of the old ballast-pit the Eocene strata are 

 highly inclined. It is, however, so extremely difficult to obtain 

 accurate dips in these deposits, owing to the occurrence of piping on 

 a scale which I have never seen equalled, that it is unsafe in the 

 present state of the section to express any confident opinion as to the 

 exact relation of the Bagshot strata to the Ridgeway disturbance. 



Though it may be impossible readily to prove by stratigraphical 

 evidence the overlap of the Lower Bagshot Beds, yet the composition 

 of the gravels demonstrates unmistakably this discordance. One 

 finds on analysis that the gravels contain in the first place abundance 

 of Chalk-flints. Next in abundance come numerous fragments of 

 the Greensand chert already mentioned. The pieces are usually 

 subangular, and of all sizes up to a foot in diameter ; so they are 

 not likely to have travelled far. The chert is probably derived 

 from the Upper Greensand. 4 Numerous pebbles of vein-quartz, 

 mostly under an inch in diameter, next attract one's attention, 

 and it is not improbable that these may come from conglomeratic 

 seams in the Weal den strata of the immediate neighbourhood. 

 Eragments of Purbeck marble, sometimes silicified, are fairly common, 

 and are associated with cherts and grits probably also of Purbeck 

 age. All the rest of the material consists of subangular veined grits, 

 hard sandstones, quartzites, quartz, radiolarian chert, and red and 

 green jaspers ; in fact, of hard siliceous material such as might be 

 derived from the weathering of the Permian breccias of Devon. 

 Black grit with small quartz-veins is abundant, and, like the radio- 

 larian chert, suggests the Culm Measures as its source, though 

 probably it also is derived secondarily through the Permian breccias. 

 Budleigh Salterton Triassic pebbles are, however, entirely missing. 



This peculiar composition of the gravels ought to give us a clue 

 to the amount of denudation that had then taken place, and also to 

 the direction in which the river flowed. The gravels of the Reading 

 Series, containing Chalk-flints and Greensand chert, suggest that 

 erosion at that period had only reached down to the Upper Green- 

 sand ; though even this amount of erosion points to a distinct post- 

 Cretaceous upheaval in the neighbourhood, which tilted the Chalk 

 and brought Greensand within reach of the eroding agent, before the 

 deposition of the Reading Beds. 



During, or before, the Bagshot period there seems to have 

 occurred another era of local disturbance, during which both 



1 In lit. [since published in Geol. Mag. for June 1896]. 



2 Quart. Journ. Geol. Soc. vol. xxxi. (1875) pi. i. fig. 2. 



3 Hid. vol. li. (1895) pi. xviii. 



4 [Chert of identical character has since been found in the Upper Greensand 

 near Abbotsbury. — July, 1896.] 



