R. M. Brydone — The Belemnitella tnucronata Zone. 353 



var.. subconicus characteristic of the mucronata zone. If this zone 

 occurs at all in the section, there would be room for a minimum 

 thickness of some 80 feet of it. 



The two places at which no palseontological grounds are given for 

 cutting out the mucronata zone altogether are at Burnt House (north 

 of Garretts) and at the west end of Bembridge Down. It will be 

 obvious on inspection that these disappearances of the zone are due 

 solely to the thicknesses assigned to the other Upper Chalk zones 

 exhausting all the available space between the Chalk E-ock and the 

 Tertiary boundary as mapped. These thicknesses appear at the 

 points in question to be purely arbitrary ; indeed, at the end of 

 Bembridge Down the c[uadratus zone seems to have been deliberately 

 given a special local increase of thickness but for which some 

 mucronata chalk must have been shown at that point. Unexpected 

 results obtained by such free methods of mapping seem to be much 

 in need of testing before any authority is claimed for them. The 

 prospect of being able to apply any test (except by making a special 

 excavation) are naturally very small if Dr. Howe found no exposure 



Pig. 1. — Tracing of a very small portion of a folding map (plate D), marked 

 on map " Garretts to Arreton Down " as " Downend Chalk Pit (21) " on 

 Gallows Hill. Illustrating Dr. Arthur W. Eowe's paper on "Zones of 

 the White Chalk of the Enghsh Coast", part v, "The Isle of Wight" 

 (see Proc. Geol. Assoc, vol. xx, pt. v, p. 209, 1908). 



at these points, but it so happens that in one of the cases I have been 

 able to do so and to obtain a result. At Burnt House I recently 

 detected a low face of massive chalk about 6 feet behind the out- 

 buildings (a shed with pigstyes at the back) shown on the map. In 

 this face I was fortunate enough to find a specimen of Belemnitella 

 mucroiiata and a small J^chi7ioGorys,-w\\i(ih^ although too much crushed 

 for any certainty, appeared likely to be the var. suhconictis. This 

 evidence is sufficient justification for definitely identifying the 

 mucronata zone at this point, which is about 150 feet from that zone 

 as mapped. If the upper boundary of the quadratus zone be rectified 

 to this extent (and it may well be that a more extensive rectification 

 is due), it will at once be clear that only an improbable disturbance 

 would carry the quadratus zone up to the Tertiary boundary just 

 west of Burnt House, as is represented. Tbe same rectification 

 would presumably have to be made in the Marsiipites plus TJintacrinus 

 zone, and this would cancel the arbitrary expansion which has been 

 given to the corangummn zone at this point. 



It tlierefore seems as if no reliance can be placed on the fluctua- 

 tions of these zonal boundaries except where they are based on the 



DECADE VI. — VOL. V. — NO. VIH. 23 



