308 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Jan. 18, 



of deposits, from the coast of Yorkshire to the cliffs of Dorsetshire. 

 Everywhere evidence can be found of the wasting action of the sea 

 on the Oolitic strata before the deposition of the superincumbent 

 rocks, and sometimes evidence of the movement of the sea-bed (to 

 which, perhaps, the watery agitation was due). In the country near 

 Oxford, and from this point south-westward, the Portland Oolite 

 has been thus greatly wasted, so as to remain in only a few detached 

 masses. There seems reason to suppose that movements of the sea- 

 bed of considerable extent followed the deposition of the Oxford Clay; 

 for the Coralline Oolite fails, and the Kimmeridge Clay grows thin, 

 and hardly traceable far from the Shotover Hills, in a direction to- 

 wards the north-east. 



The deposition of the Cretaceous series on the wasted Oolites was 

 thus inevitably irregular ; but in addition we have the varieties of 

 littoral, estuary, and fluviatile deposits on the boundary- surface of 

 the Oolites ; great surface -waste, referable to the Postpliocene age ; 

 and faults which seem to be of great effect, but are not yet traced 

 out. 



Under these circumstances, it is at once a very interesting and a 

 very perplexing problem of field-geology to trace out the detached, 

 unconformed, and wasted cappings of sand and ferruginous stone 

 which, in several places, lie on the Kimmeridge Clay, and are not 

 themselves covered by strata of more definite character and age. 



The conclusions derived from the field-surveys will not be satis- 

 factory until fully supported by the evidence of fossils scrupulously 

 collected by careful hands. 



One of the most interesting of the sections near Oxford is seen at 

 Culham, on the northern bank of the Thames ; and this may be 

 compared with another in the line of railway near Culham Station, 

 about a mile to the north-east, with a hill-capping at Toot Balclon 

 and a cliff-section at Clifden. I have been in the habit of taking 

 my class to some of these localities for several years. 



On entering the excavation at Culham we perceive about 40 feet 

 of level-surfaced clays and sands, under a cover of flint- gravel mixed 

 with worn shells of Gryphcea dilatata and other spolia of the adja- 

 cent country. Nearly the whole mass of the clays and sands exca- 

 vated here is employed for brick-making; and the digging-operations 

 mix them much together. A slight glance at the section presents 

 enough of uniformity to induce the belief that the whole might 

 belong to one continuous deposit. If, under this impression, a pa- 

 laeontologist viewing the excavation should pick up Thracia depressa 

 and Gardiwm striatulum, and obtain from the workmen teeth oiPlio- 

 saurus, he will probably write Kimmeridge Clay on the whole section. 

 Another geologist, arriving when the clay is not being dug, may 

 examine a different part of the deposit and find Ammonites dentatus 

 and Belemnites minimus, and may colour on his map, undoubted Gaxdt. 

 But when, instructed by several visits, the whole section is clearly 

 made out, we find two clays in the pit, of entirely different geological 

 age, separated by a bed of sand apparently conformed to each — so 

 far as this very limited area gives any evidence. 



