September 15, 1898] 



NA TURE 



479 



mention papers by Mr. H. B. Woodward and Mr. Beeby 

 Thompson, each in explanation of the arborescent figures in the 

 Cotham Marble. The latter revives an old idea with modifica- 

 tions, and his theory certainly seems plausible. Mr. H. B. 

 Woodward's Memoir of 1893 <3oes full justice to the Lias of 

 this district, and much original matter is introduced. 



It is, however, in the Inferior Oolite that the most important 

 interpretations have tobe recorded since the days when Dr. Wright 

 and Prof. J. Buckman endeavoured to correlate the development 

 of the series in the Cotteswolds with that in Dorset. To this subject 

 I alluded at considerable length in my address to the Geological 

 Society in 1893, pointing out how much we owed in recent 

 years to the late Mr. Witchell and to Mr. S. S. Buckman. In 

 the following year appeared Mr. H. B. Woodward's Memoir 

 on the Lower Oolitic Rocks of England ("Jurassic Rocks of 

 Britain," vol. iv.), wherein he did full justice to the work of 

 previous observers. Meantime Mr. Buckman has not been 

 idle, and his paper on the Bajocian of the Sherborne district 

 {Quart. Journ. Gcol. Soc, vol. xlix., 1893, p. 479) marks the 

 commencement of a new era, where the importance of minute 

 chronological subdivisions, based upon the prevailing am- 

 monites, is insisted on with much emphasis. This system he 

 considers to be almost as true for the Inferior Oolite as for the 

 Lias. 



There can be no doubt that its application has enabled Mr. 

 Buckman to effect satisfactory correlations between the very 

 different deposits of the Cotteswolds and those of Dorset 

 and Somerset. In subsequent papers also he brings out an 

 important physical feature, viz. the amount of contemporaneous 

 denudation which has affected deposits of Inferior Oolite age in 

 this country. This serves in part to explain the absence of well- 

 known beds in certain areas. For instance, in the Cotteswolds 

 contemporaneous erosion has, prior to the deposition of the 

 Upper Trigonia-gnt, cut right through the intervening beds, so 

 as to produce in the neighbourhood of Birdlip a shelving trough 

 6 miles wide and about 30 feet deep. Thus the extensively 

 recognised overlap of the Parkinsoni-zoviQ is accentuated in 

 many places. 



We have a further instance of good work in the case of Dundry 

 Hill. An inspection of the i-inch Survey map would lead one 

 to suppose that the Inferior Oolite there rests directly on the 

 Lower Lias. Recently, owing to the investigations of Messrs. 

 Buckman and Wilson,^ this apparent anomaly has been removed, 

 whilst beds of Middle and Upper Lias age, and even Midford 

 Sands have been recognised. In this way the authors claim to 

 have reduced the thickness assigned to the Inferior Oolite on 

 Dundry Hill by about 100 feet. In the paper above quoted the 

 vicissitudes and faunal history of the Inferior Oolite from the 

 cpalimts-zont to the Parkinsoni-zone inclusive are shown with 

 much detail ; whilst the position of the chief fossil-bed in time 

 and place has been well established. The general resemblance 

 of the Dundry fossils to those of Oborne, which I could not fail 

 to notice in working out the Gasteropoda of the Inferior Oolite, 

 now admits of explanation. Although the quondam Humphri- 

 esmtius-zone is richly represented, yet the particular Humphri- 

 €sianum-hemeTa. is held to be absent at Dundry. But if there 

 is a Sowerbyi-htA anywhere it should serve to connect these two 

 localities, where, according to Mr. Buckman's phraseology, the 

 principal zoological phenomenon is the acme and paracme of 

 Sonnininre. 



Mr. Buckman, as we have seen, is no longer satisfied with the 

 old-fashioned threefold division of the Inferior Oolite, and his 

 time-table includes at least a dozen hemerae, with prospect of 

 increase. Granting that it would have been difficult to solve 

 the Dundry problem without a detailed knowledge of ammonite 

 horizons, there arises the question as to the utility of such 

 minute subdivisions for the purposes of general classification. 

 Mr. Buckman has earned the right to put forwards, if he pleases, 

 the several stratigraphical rearrangements in which from time 

 to time he indulges. The Inferior Oolite has been his especial 

 playground, and, as the kaleidoscope revolves, this formation 

 is perpetually made to assume different proportions, even to the 

 verge of extinction. But this practice is not without its dis- 

 advantages ; whilst the invention of new names tends to clog 

 the memory, and the novel use of old ones is apt to produce 

 confusion. 



We have not quite finished with Dundry yet, since that classic 



1 Quart. Journ. Geol. Soc. vol. Hi., 1897, p. 669. Cf. also Proc. lirist. 

 Nat. Soc, vol. viii., 1897, pt. ii. p. 188. 



NO. 1507, VOL. 58] 



hich 

 rict, 



hill serves to illustrate in Mesozoic limes a peculiarity of w! 

 I have already pointed out two notable instances in this dist 

 where an abrupt and seemingly unaccountable difference is 

 observed in beds which are approximately synchronous. The 

 problem to be solved is this— why does the fossiliferous portion 

 of the Inferior Oolite on Dundry Hill resemble that of the 

 neighbourhood of Sherborne, both in lithology and fossils, 

 rather than that of the Cotteswolds, only a few miles distant ? 



Nine years ago Mr. Buckman offered an ingenious solution of 

 this difficulty (Proc. Cottes. Club, vol. ix., 1890, p. 374), 

 though his recent investigations at Dundry, and especially his 

 appreciation of the effects of contemporaneous erosion, may have 

 caused him to alter his views. Like most people who wish to 

 account for strong local differences, he placed a barrier of 

 Palaeozoic rocks between Dundry and the southern prolongation 

 of the Cotteswold escarpment. At that time it was not fully 

 realised that the Inferior Oolite in the Bath district is, for the 

 most part, limited to the Parkinsoni-zont, so that the comparison 

 was really being made between beds of different age as well as 

 different physical conditions. The question resolves itself into 

 one of local details, which are not suited for a general address. 

 Still, I think it may be taken for granted that, notwithstanding 

 the east-and-west barrier of the Mendip range, which acted 

 effectually previously to the Paritnsom-oveTla.p, there was in 

 some way a communication by sea between Dundry and Dorset- 

 shire, more especially during the Sowerdyi-stage, and this most 

 probably was effected round the western flank of the Mendips. 

 Thus, without acceding to the necessity for a barrier facing the 

 southern Cotteswolds, we may readily believe that much of the 

 Inferior Oolite of Dundry Hill is to be regarded as an outlying 

 deposit of the Anglo-Norman basin. If this be so, it is difficult 

 to avoid the conclusion that the low-lying area of the Bridge- 

 water flats was, during part of the Inferior Oolite period, 

 occupied by a sea which was continuous from Sherborne to 

 Dundry, and that, although the barrier of the Mendips was 

 interposed, communication was effected round the west flank of 

 that chain. This would make a portion of the Bristol Channel 

 a very ancient feature. 



We must now take a wide leap in time, passing over all the rest 

 of the Jurassics, and just glancing at the Upper Cretaceous system, 

 which reposes on the planed-down surface of the older Secondary 

 rocks. The remarkable double uncomformity is nowhere better 

 shown than in the south-west of England. Some of the move- 

 ments of the older Secondary rocks, prior to the great revolution 

 which brought the waters of the Cretaceous sea over this region, 

 have been successfully localised by Mr. Strahan, more especially 

 in the south of Dorset. 



Owing to Tertiary denudation the Chalk in this immediate 

 district has been removed, and we have no means of judging the 

 relations of the Cretaceous deposits to the Paleozoic rocks of 

 Wales. If we may judge by results recently recorded from 

 Devonshire {cf. Jukes-Browne and Hill, Quart. Journ. Geol. 

 Soc. vol. Hi., 1897, p 99), the Lower Chalk especially undergoes 

 important changes as it is traced westwards, and generally 

 speaking terrigenous deposits seem more abundant in this 

 direction. At the same time the more truly oceanic deposits, 

 such as the Upper Chalk, appear to be thinning. As regards the 

 possible depths of the Cretaceous sea at certain periods, we are 

 supplied with some interesting material in Mr. Wood's two 

 papers on the Chalk Rock {Quart. Journ. Geol. Soc..,yo\. Hi., 

 1897, p. 68, and vol. liii., 1898, p. 377), which has been 

 found especially rich in Gasteropoda at Cuckhamsley, near 

 Wantage. 



Tertiary, Pleistocene, and Recent. — Although the Tertiaries 

 of the Hampshire basin are within the " Index-map" which we 

 have been considering, they may be regarded as beyond our 

 sphere. Some of the gravels of Dorsetshire, which have gone 

 under the name of plateau gravels, are held by Mr. Clement 

 Reid to be of Bagshot age. Many of the higher hill gravels 

 most likely date back to the Pliocene, and even further, and 

 represent a curious succession of changes, brought about by 

 meteoric agencies, where the valley-flat of one period, with its 

 accumulated shingle, becomes the plateau of another period — 

 an endless succession of revolutions further complicated by the 

 Pleistocene Cold Period, which corresponds to the great Ice 

 Age of the north. 



In the more immediate neighbourhood of Bristol, since some 

 date in Middle Tertiary time, the process of earth-sculpture, 

 besides laying bare a considerable amount of Palaeozoic rock. 



