^86 Correspondence — R. M. Brydone. 



sciences. The 'zone of Marsnpitcs,' to wliicli Mr. Jukes-Browne 

 appeals, is one of the finest possible examples of what I mean. The 

 ' zone oi^Marsupites,'' introduced by Barrois, was an international zone. 

 It was adopted from a country in which not much was known about 

 it, and it had not even received a separate name ; its upper boundary 

 for England was wholly' undefined, its lower boundary was only 

 defined in the IMargate area by a physical character which Barrois 

 only thought he recognised again in the Sussex area, and there 

 wrongly according to Dr. Rowe, and the type fossil only occurs in 

 about 40 feet out of some 300 feet attributed to this zone by Barrois. 

 I do, though I gather Mr. Jukes-Browne will disapprove, most 

 strongly urge the limitation of the ' zone of Marsitpites,' for the south 

 of England at any rate, to the important bed of very uniform thickness 

 and position in the series which contains Blnrsnpites '' in every foot," 

 and outside which Marsnpites is practically non-existent. Can there 

 be any doubt as to which of these two zones is the more logical and 

 practically useful ? 



Now it so happens that the international zones introduced by 

 Barrois up to and including the zone of M. cor-testudinariitm are 

 sound provincial zones for the south of England, answering (except 

 in the case of the zone of B. plena) very well to what Mr. Jukes- 

 Browne considers such an unreasonable test, i.e. the occurrence of the 

 type fossil in every foot of the zone. But that does not make it any 

 the less desirable if we are establishing a new zone to establish the 

 most accuratelj'^ defined one that we can. Does Mr. Jukes-Browne's 

 zone of 0. Jnnata satisfy the reasonable requirements of scientific 

 accuracy ? How would he define its upper and lower boundaries ? 

 Clearly not by the appearance and disappearance of 0. hoiata, for 

 there are at least 10 feet of chalk exposed at Trimmingham below 

 the lowest occurrence of this species and anything from 25 feet 

 upwards above the highest occurrence, and 1 cannot see any other 

 possible criterion. The zone of ' TerehratuUnce ' which I propose 

 begins where TerehraiuJina gracilis appears (and I am in hopes of 

 satisfying myself that this is just above the hard yellowish bed, in 

 which case the zone of 2>. imicrouata would have at Trimmingham 

 an upper boundary defined both physically and palceontologically), 

 and will end where T. gracilis disappears, unless before that point 

 is reached some other fossil worthy of being made a zone fossil 

 comes in. The only objection I can see to my zone of Terehratulinee 

 will be removed when the characteristic fossil of the zone below 

 that of n. planus is properly named, and I hope we shall not have 

 to wait much longer for this. 



R. M. Brydone. 



16, South Avdley Stkeet, W. 



[Erratum. — In Mr. R. M. Brydone's article in February number, 

 Geol. Mag., 1906, p. 77, line 33 from top of page, for 'blending' 

 read ' banding.' — Edit. Geol. Mag.] 



