Correspondence — R. M. Brydone. 285 



fishes, etc. ; (7) tests of entomostraca ; (8) quartz in subangular 

 and well-rounded grains ; (9) black granules, apparently of iron- 

 oxides ; (10) rich-brown, polished phosphatic concretions ; and 

 (11) dull-green grains, imperfectly rounded. These constituents are 

 enumerated as nearly as possible in order of abundance. 



The Winterbourne-Boxford phosphates have a known range in 

 time considerably greater than those of Taplow. Their advent far 

 down in the cor-anguinum zone is especially interesting ; for in 

 England, as Mr. Jukes-Browne has remarked, that subdivision of 

 the Chalk almost everywhere "presents the appearance of having 

 been quietly and continuously accumulated in water that was seldom 

 disturbed by bottom currents," albeit a tendency to develop hard 

 bands at one horizon, at least, is apparent in the western part of 

 the London Basin. The Phosphatic Chalks of Winterbourne and 

 Taplow evidently mark places on the sea-floor particularly liable to 

 the impingement of strong currents, and may mark places above 

 which the water commonly had a gyratory motion. In any case, 

 their zonal range argues a marked degree of stability in the current- 

 system of the body of water in which they were laid down. 



coI^I^Es:po:ITIDEl;Tc:El. 



THE TEIMMINGHAM CHALK AS A ZONE. 



SiK, — I hope you will allow me at this late hour to reply to 

 Mr. Jukes-Browne's letter in your February number. It raises some 

 very large questions which cannot be adequately dealt with in a 

 letter, but I should like to define my position. 



It appears to me that the term ' zone ' is applied indiscriminately 

 to at least two different conceptions. One of them is what may be 

 labelled the 'international zone.' This covers the great ill-defined 

 subdivisions which persist over large areas embracing more than one 

 natural province, and by which we correlate the equally ill-defined 

 local territorial names such as ' Norwich Chalk,' ' Maestricht Chalk,' 

 ' Meudon Chalk,' etc. These correlations are interesting, but of little 

 practical value, as the zones themselves are rarely, if ever, accurately 

 defined. 



The other conception above referred to is what may be called the 

 ' provincial zone.' Each of these zones contain such a thickness of 

 sediment as from place to place over a natural province (whether 

 made natural by modern geographical or ancient geological conditions) 

 contains tJaoughout {pace Mr. Jukes-Browne) some well-marked form 

 which is comparatively scarce or altogether absent both above and 

 below. These zones have upper and lower limits which are well 

 defined either palaeontologically or stratigraphically, often in both 

 ways. They are therefore capable of practical application by the 

 pit- worker, who most stands in need of help, and through him by allied 



