Corres23ondence — Mr. Philip Lake. 89 



fossils recorded in England, with the range of each. A consideration 

 <if this list enables the author to suggest tliat the lower limit of the 

 formation should be drawn at the first evidence of lihgetic life after 

 'the deposition of the gypsiferous and red or green marls, which (at 

 any rate in their lower part) are certainly of Keuper age. The upper 

 limit may, for convenience, be drawn at an indefinite level where 

 Modiola minima and Fleuromya crowcombeiana become very rare, and 

 the ammonitic and Liassic fauna begins. Further discussion of the 

 lithoiogical, physical, and palseontological evidence leads the author 

 to recognize that the affinities of the Rhastic, thus defined, are rather 

 with the Jurassic rocks than with the Trias. The following zones 

 are suggested, in descending order : — 



Zone of Fleuromya croiucomheiana = White Lias. 



,, Monotis decussata = Gotham Marble and just above. 

 , , Estheria mii/uta var. Brodieana, and Naiadita. 

 ,, Pecten valoaiensis. 



,, Aviciila eontorta = Black Shales and a limestone bed. 

 ,, Bone-bed. 



These zones seem to be fairly constant throughout England, and 

 harmonize well with those of Germany, although they cannot be 

 expected to fit in with the oceanic type of the Alps and the 

 Mediterranean. Further consideration shows that the fossils give 

 evidence of migration, but very little of evolution. The paper closes 

 ■with the description of a new species of Anomia and a bibliography. 



GOI^I^:E]SI=OI^^x)SI^^G:E. 



ATMOSPHEEIC EROSION IN CORSICA. 



Sir, — The remarkable mode of erosion described by Mr. Tuckett 

 in the Geological Magazine for this month is not uncommonly met 

 with in the drier regions of the globe, and excellent examples are 

 described and figured by Walther in his " Die Denudation in der 

 Wiiste" (Abh. k. sachs. Ges. Wiss., Math.-Phys. Classe, 1891) and 

 "Das Gesetz der Wiistenbildung " (Berlin, 1900). Fig. 7 in the 

 latter work presents a particularly close resemblance to the Tete de 

 Ohien. It is a reproduction of a photograph taken near the Indian 

 desert. 



Walther attributes the peculiar mode of erosion in these regions 

 to the relative persistence of dew and other moisture on the shady 

 side of the boulder or cliff, and its rapid evaporation on the sunny 

 side. The shaded side consequently weathers much more quickly 

 than the other, and the weathered material is removed by the wind. 



In the Northern Hemisphere the cavities formed are generally, 

 though not always, on the northern or western side of the rock ; but 

 from the shadows shown in Mr. Tuckett's beautiful photograph of 

 the Tete de Chien, I infer that in this case the cavity does not face 

 the north. 



It would be interesting to learn whether the Corsican examples 

 -support Professor Walther's view. Philip Lake. 



13, Park Street, Cambridge. 

 January 15th, 1904. 



