158 F. L. Kitcliin c& J. Pringle — The Upper Gault in 



It must be understood, in the first place, that to obtain sound 

 results in an investigation of this character the fullest reliance must 

 be placed on the value of palaeontological evidence. Indeed, it must 

 be insisted that in the study of the Cretaceous, as of other stratified 

 rocks, fossils, when obtainable, provide the most important part of 

 the whole evidence upon which sound stratigraphical conclusions 

 can be based. In any attempt to determine the zonal relationships 

 when studying deposits of clay such as the G-ault, subject to lateral 

 change to other types of sedimentation, we must be prepared to pin 

 our faith entirely to the fossils. There remains, of course, the 

 recognition of the species appropriate for such an important use and 

 the power to apply this evidence legitimately, in the scientific sense. 

 These are obviously matters depending entirely on the uniformity 

 of previous experience and the competence of the individual 

 investigator. We would not lay emphasis on preliminaries so trite 

 were it not that in this country we still witness the publication of 

 fallacious stratigraphical correlations that have resulted from 

 failure to utilize the means at hand for arriving at a true 

 interpretation. 



The Gault Clay is usually poorly exposed, so that the investigation 

 of its characters and relations, except at isolated localities, often 

 distantly separated, is frequently a matter of much difficulty. In 

 some districts, as in the Isle of Wight, there is a further obstacle 

 in the masking and confusion caused by the slipping of the clay on an 

 extensive scale. It follows, then, since exhaustive fossil-collecting 

 can seldom be carried out, that it is necessary to use to the best 

 stratigraphical advantage such fragmentary evidence as may be 

 provided by the few fossils obtainable in obscure surface-exposures 

 or in wells and borings. The chief requirement in the present 

 connexion is to be able to determine with certainty whether the 

 lowest member of the Gault seen at any given locality represents 

 the true basal part of the Gault or the lower part of the Upper Gault. 

 Only by ability to do this is it possible to avoid being misled by 

 delusive stratigraphical appearances. 



The true basal part of the Gault Clay, the so-called interrupt us- 

 zone, is highly fossiliferous, and always contains numerous well- 

 ribbed hoplitid ammonites showing pronounced peripheral sulcation. 

 These are frequently preserved in smooth phosphatic nodules with 

 pale surfaces. The keeled ammonites are absent. We have found 

 that the interruptus-Gault, wherever it has been seen in this country, 

 rests always upon the ma7nmillatus-hed. An unconformable junction 

 with older strata may occur locally below that bed, as in North Dorset, 

 but never at the base of the interruptus-zone. So invariable is this 

 unity of the interrujotus-Geiult and its substratum that the direct 

 superposition of Gault Clay or its equivalent upon any rock older 

 than the mammillatus-hed suffices to negative the presence of the 

 interruptus-zon e . ^ 



' Cf. C. Barrois, RecJierches sur le Terrain Critaci Supdrieur de 

 I' Angleterre et de VIrlande, 1876, p. 7. 



