On Dr. Roive's Zones of the White Chalk. 173 



formerly had been approximation. He has not confined himself to 

 re-describing the zones with uncertain limits, but with fine analysis 

 he distinguishes a succession of beds which furnish fixed datum-lines 

 — such as the " Bed well line" in the zone of Uintacrinus, the 

 ^'■Ammonites leptophyllus bed" (where he mentions 105 examples), 

 the " Sponge bed " forming the base of the Uintacrinus zone, the 

 '■'■ JEchinocorys pyramidatus bed," the '■'■ Echinoconus bed" formed of 

 a carpet of this species, the " Whitaker 3-inch fiint-tabular " band 

 towards the top of the Micraster cor-anguinum zone, etc. Thus he 

 takes under consideration at the same time the lithological and the 

 palgeontological characters ; the former, though more easy to recognize 

 at first sight and having the advantage of furnishing at each point 

 geometric limits, but failing from want of permanency from place to 

 place. The author has rendered lasting service in fixing the distances 

 which separate the absolute datum-lines, the limits assigned to his 

 life-zones. Whether the name-fossils chosen for these zones are 

 suitable is open to discussion, as it is well known that the range of 

 few narae-fossils are confined to one zone. No one has ever relied 

 upon one fossil for obtaining a zonal determination, and though the 

 name-fossil attains its maximum development in the zone to which its 

 name is applied, we rely more upon a group of associated forms 

 peculiar in certain characteristics or in themselves as our zonal guides. 

 It is this association of life-forms and their variations, as we trace 

 them zone by zone, which gives to zonal geology its value. 



Sussex. 



The collection of fossils, easy and amusing though it be at certain 

 points of the Chalk cliffs, becomes, unfortunately, exacting at other 

 points where one proposes to base an analysis of the section on their 

 distribution. Thus, in the Beachy Head section the author was forced 

 to rely entirely on the genus Micraster for limiting the zones, mapping 

 out the chalk foot by foot, and taking the specimens obtained from 

 each foot of chalk to the water's edge and there cleaning and 

 determining them before proceeding to the next foot. It becomes 

 very difiicult to collect this genus of fossils when they are rare or 

 when they are shorn off by the battering action of the shingle at the 

 foot of the cliff, or in those dangerous places where one has to pass 

 hurriedly whilst fragments of cliff are falling from above, or the sea 

 threatens to cut off one's retreat below. The description of Beachy 

 Head gives a fair example of the perseverance of the author in search 

 of his Junctions, for there he uses wreckage to increase his reach up 

 the cliff ; or that of Seaford Head, where he descends roped at the 

 point where he has noticed the fossils he is in search of by the calcite 

 fracture of their tests, afterwards carefully extracted by the knife 

 from the face of the battered clifi:. Hence those geologists who have 

 described the Seaford cliffs before him must not be astonished when 

 comparing their work with his to find it considerably perfected. 



Borset. 



The whole of this coast bristles with difficulties, but in spite of 

 altered and consequently barren rocks the author has been able to fix 



