460 Dr. C. Callaivay — On River Curves. 



in the Albert Museum, but has not found any trace of Belemnites 

 or of fossils characteristic of the zones of Actinocamax quadratus 

 or Belemnitella mucronata. Of course this is negative evidence, 

 but where Belemnitella is common in Chalk, as near Norwich, casts 

 of its alveolar cavity are common in the flint-gravels, and one would 

 expect the remains of this fossil to occur in the Haldon gravel 

 if the zone of Bel. mucronata had been under erosion in Devonshire 

 during the time of the Bagshot Beds. It should, however, be 

 remembered that even if no fossils belonging to higher zones than 

 that of Marsupites should ever be found on Haldon, their absence 

 will not mean more than the absence of such zones within the area 

 at the time when the Bagshot Beds were formed. It will not prove 

 that the higher zones never extended into Devonshire, but simply 

 that they were destroyed during the upheaval of the Chalk and 

 before the formation of the Bagshot Beds. 



The occurrence of the plates of Marsupites does, however, enable 

 us to form an estimate of the total thickness of Chalk which may 

 have existed in the neighbourhood of what are now the Haldon 

 Hills when the Bagshot gravels were being accumulated. The 

 Lower Chalk would be represented by only a few feet of calcareous 

 sandstone, as on the Devon coast, and may consequently be neglected ; 

 the Middle Chalk is about 100 feet thick at Beer, and the mean 

 thickness of the zone of Holaster planus in Devon is about 40 feet. 

 To get an estimate of the thickness of the zones of Micraster 

 cortestudinarium and M. coranguinum we must go to Dorset, where 

 the mean of several measurements gives 80 feet for the former and 

 200 feet for the latter.^ Finally, the thickness of the zone of 

 Marsupites was found to be 111 feet near White Nothe, and Marsupites 

 are only found in the upper 30 feet ; if this depth was maintained 

 to the westward, the total thickness of Chalk under erosion in 

 Devonshire would be as follows : — 



feet. 

 Zone of Marsupites testudinarius ... ... ... ... Ill 



Micraster coranguinum 

 ,, Micr. cortestudinarium 

 ,, Holaster planus ... 

 Middle Chalk 



200 

 80 

 40 



100 



We may therefore safely assume that the Haldon gravels represent 

 the riddlings of about 500 feet of Chalk. 



IV. — On a Cause of Eivbr Curves. 

 By C. Callaway, D.Sc, F.G.S. 



IN recent work on the laws of river-dynamics an important part 

 has been taken by members of the Cotteswold Naturalists' Field 

 Club, Dr. T. S. Ellis,^ in 1882, announced the suggestive fact that 

 the course of all the eastern tributaries of the Severn between 



1 See Dr. A. W. Rowe oa " The Zones of the White Chalk of Dorset," in Proc. 

 Geol. Assoc, 1901, vol. xvii, p. 11. 



- "On some Features in the Formation of the Severn Valley." Read before the 

 School of Science Philosophical Society, Gloucester. The geological world may 

 perhaps be excused for overlooking the publications of this Society. 



