176 Rev'mcs — Dr. Howe's Zones of the White Chalk. 



650 feet for Act. verus ; while Cardiastcr ananch/tis has been traced for 

 640 feet and Infulaster rostratus for nearly 700 feet outside the limit 

 of their ordinary zones. In point of fact this extension of known 

 ranges for fossils is one of the most conspicuous results of the work in 

 this area. 



Isle of Wight. 



Whoever visits the Isle of Wight loves to return, but he who has 

 studied its geology cannot leave it. The memoir by Dr. Rowe is fresh 

 proof of this, since after having studied the cliffs of the island he was 

 moved to examine the interior. We owe to this circumstance, and to 

 his association with C. D. Sherborn, a geological map of six inches to 

 the mile, unique of its kind, on which eight zones are defined with 

 precision in a mass of Chalk apparently uniform. These zones show 

 from one extremity to the other of the island variations in thickness, 

 contemporaneity of deposit, and varied faunas, which have furnished 

 information as to the migration of species. The zone of B. mucronata 

 shows considerable differences in its thickness, indicative of important 

 pre-Tertiary subaerial denudation. 



In this island the Chalk is seen in its greatest thickness and most 

 uniform sedimentation. The principal modification observed is in 

 relation to the mechanical forces which tilted the beds ; the Chalk 

 is there hardened, and the flints are crushed as one passes from north 

 to south in proportion to the inclination of the strata. 



This memoir is in advance of those of previous writers by reason of 

 the exactness of the stratigraphical observations ; it is in advance of 

 the previous memoirs of its author in the revision of the list of fossils, 

 which gives detailed information on zoological groups like sponges, 

 corals, bryozoa, and annelids, which have up to now been neglected. 



The mass of the White Chalk, 1,500 feet in thickness, of uniform, 

 slow, placid, and uninterrupted sedimentation, taking lAace over vast 

 periods of time and over a large extent of country, allows us to follow 

 every stage in unbroken continuity in the evolution of a genus and 

 the equally interesting zonal variations in a species. Studies such as 

 these afford the surest contribution to our knowledge of the evolution 

 of fossil forms. They differ widely from the ordinary systematic 

 papers where so many new species are established on isolated or 

 unsatisfactory specimens, providing so much difficulty and uncertainty 

 in future identification. The author of these memoirs, instead of making- 

 new species, has followed the variations of the different typical 

 Micrasters throughout the successive horizons and established series of 

 individual forms, besides distinguishing the mutations in the successive 

 beds from one variety to another of the same specific type. 



He has shown that not only is the zonal theory correct, in that at 

 certain levels of the Chalk there exist fossils which are either rigidly 

 restricted to one particular zone (e.g. Inoceramus lahiatus), or that 

 certain groups of guide-fossils, though not so restricted, are by their 

 association equally characteristic of horizon ; but he has also shown 

 that certain fossils vary so markedly in shape or other essential 

 features, as they range from a lower to a higher level, that we can 



