Reviews — Dr. Rotce on the White Chalh of Yorkshire. 231 



named form appeal's not yet to liave been found at all in Yorkshire, 

 will meet with the approval of every local worker. 



The choice of Infitlaster rostratus to serve a similar purpose for 

 the zone of Micraster cor-anguinum is, however, open to question, 

 although when first suggested it seemed to the present writer to 

 be well adapted. But its range has been so greatly extended by 

 Dr. Eowe's researches, both above and below the belt in which it is 

 most abundant, and with which its name is now associated, that its 

 unsupported presence seems inadequate to determine the zone, and 

 we should feel less confident than the author in assigning a small 

 inland pit " without hesitation " to the zone of Micraster cor-anguinum 

 on the strength of the discovery of this fossil alone (p. 233). 



And here we may note that in respect to Dr. Eowe's demarcation 

 of the zone of Micraster cor-nngninnm there appears to be a certain 

 arbitrariness, perhaps unavoidable but still unsatisfactory, especially 

 since the zone as now defined is made to bestride the only lithological 

 line traceable in the Yorkshire Chalk, namely, that separating the flinty 

 from the fiintless Chalk. Indeed, with regard to several of the zonal 

 boundaries Dr. Eowe will no doubt himself be ready to allow that in 

 these Yorkshire sections, when the evidence is often so imperfect, the 

 chosen line reflects in its particular location an opinion or deduction 

 rather than an absolute fact, even though it represent the best con- 

 ventional line that is likely to be attained. The position is precisely 

 that in which the mere stratigrapher often finds himself in tracing 

 boundaries that he knows to lie within certain limits but to be in- 

 determinable within these limits. And just as the stratigrapher's line 

 when drawn on the map sometimes gives an unwarranted impression 

 of finality, so may these zonal boundaries if too rigidly interpreted. 



One important deduction to be drawn, then, from Dr. Eowe's 

 experiences in the Yorkshire Chalk — a deduction that has also 

 impressed itself upon the present writer iu extending the area of 

 his investigations in the Lower Cretaceous rocks — is that, although 

 the general succession of life-forms that go to the making of 

 'zones' remains constant over wide areas, the range and association 

 of individual species, however sharply defined at one spot, can 

 rarely be traced far without showing disintegration and change. 

 Thus the difficulties that beset the stratigrapher owing to gradual 

 change in the lithological character of sediments have their 

 counterpart in the difficulties that beset the zonal palgeontologist 

 in the gradual change of zoological assemblages. 



The diversity between the fauna of the southern and northern 

 Chalk has long been recognized, but it has never before been so 

 definitely formulated as by Dr. Eowe, and we regard this detailed 

 comparison as the most valuable part of his paper. For the present 

 he is content to state the differences without attempting further to 

 discuss the cause than to state (p. 280) that they afford " convincing 

 evidence of the working of variation in geographical distribution." 

 It is indeed astonishing that in such a continuous and homogeneous 

 mass as the Chalk, which seems to postulate that the physical 

 conditions of the sea-floor must have been well-nigh identical over 

 the whole region covered by the deposit, there should be this great 



