Correspondence — Mr. A. J. Jukes-Browne. 335 



must at that period have been much milder than at present. Gwyn 

 Jeffreys and Searles Wood, from the shells, considered that the 

 deposit could not be younger than Middle Ked Crag, but Mr. Starkie 

 Gardner was inclined to assign a greater age to it. Dr. Tboroddsen 

 thinks that these Crags are younger than the 'Old Basalts' of 

 Tjcirnes. The author finds, however, that, at a height of 500 feet 

 above the sea, they are overlain by the ' Eastern Basalts,' and 

 are indurated and altered by them. Thus there is a fossiliferous 

 intercalation, over 500 feet thick, occupying part of the great 

 gap between the Tertiary and the Pleistocene rocks, the latter 

 containing indurated ground-moraines. The basal layer of the 

 Pleistocene Series is fossiliferous, and has yielded 22 species of 

 mollusca, 20 of which represent a highly Arctic fauna (with Yoldia 

 arctica), such as is at the present day found living along the coasts 

 of Spitzbergen. Certain of the larger basalt dykes are cut off at the 

 base of the Crag. The absence of the Crag deposits from other 

 localities is explained by the erosion of the coastline. 



OOS.iaiESI^'OIiTXD.BlsrGE. 



THE ZONE OF OSTREA LTJNATA. 



Sir, — I have no objection to the distinction which Mr. Brydone 

 wishes to make between ' international ' and ' provincial ' zones, but 

 I must maintain my opposition to his conception of a provincial zone. 



I believe that I express the generally accepted view of such 

 a zone in briefly defining it as a band of strata characterised by 

 a special group of species. That is the definition of a zone given 

 by me in vol. i, p. 34, of the Memoir on the Cretaceous Piocks of 

 Britain, and the zone of Ostrea lunata as proposed in vol. iii of that 

 memoir is based upon that definition, the name being an index of 

 the fauna and not of the zonal limits. 



Now Mr. Brydone wants to restrict a provincial zone to a band in 

 which some type-fossil can always be found in every foot of its 

 thickness ! Moreover, he has the boldness to say that all the zones 

 introduced by Barrois in the South of England below that of 

 Marsupites cor-anguinum answer to the test he imposes. I am 

 greatly surprised that he should commit himself to such a statement, 

 for it is not true even of the Chalk of Dover, while he ought to 

 know that in Dorset Holaster subglobosus is so rare in the upper 

 part of the Lower Chalk that I have not heard of one being found. 

 Again, in some parts of Wiltshire Terehratulina lata is quite a rare 

 fossil in the Terehratulina zone. These are cases in which the 

 Chalk is almost unfossiliferous, and how can his idea of a zone be 

 applied to them ? 



He asks me how I would define the upper and lower boundaries 

 of the zone of 0. lunata. I reply, in precisely the same manner as 

 the other zones adopted in my memoir are defined, not necessarily 

 by the index-species, but by means of the fauna as a whole. It may 

 be that its base is best defined by the incoming of T. gracilis, and 



