318 ANIMAL OEIGIN OF WHITE CHALK. [Ch. XVII. 



three divisions in the south of England equals, in some places, 1000 

 feet. 



The foregoing section (fig. 279) will show the manner in which the 

 white chalk extends from England into France, covered by the ter- 

 tiary strata described in former chapters, and reposing on lower cre- 

 taceous beds. 



Geographical extent and origin of the White Chalk. — The area over 

 which the white chalk preserves a nearly homogeneous aspect is so 

 vast, that the earlier geologists despaired of discovering any analogous 

 deposits of recent date. Pure chalk, of nearly uniform aspect and 

 composition, is met with in a northwest and southeast direction, from 

 the north of Ireland to the Crimea, a distance of about 1140 geo- 

 graphical miles, and in an oppcsite direction it extends from the south 

 of Sweden to the south of Bordeaux, a distance of about 840 geo- 

 graphical miles. In Southern Kussia, according to Sir E. Murchison, 

 it is sometimes 600 feet thick, and retains the same mineral charac- 

 ter as in France and England, with the same fossils, including Inoce- 

 ramus Cuvieri, Belemnites mucronatus, and Ostrea vesicularis. 



But it would be an error to imagine that the chalk was ever spread 

 out continuously over the whole of the space comprised within these 

 limits, although it prevailed in greater or less thickness over large 

 portions of that area. On turning to those regions of the Pacific 

 where" coral reefs abound, we find some archipelagoes of lagoon 

 islands, such as that of the Dangerous Archipelago, for instance, and 

 that of Kadack, with several adjoining groups, which are from 1100 

 to 1200 miles in length, and 300 or 400 miles broad ; and the space 

 to which Flinders proposed to give the name of the Coralline Sea is 

 still larger, for it is bounded on the east by the Australian barrier — 

 all formed of coral rock — on the west by New Caledonia, and on the 

 north by the reefs of Louisiade. Although the islands in these areas 

 may be thinly sown, the mud of the decomposing zoophytes and 

 foraminifera may be scattered far and wide by oceanic currents. 

 That this mud would sometimes resemble chalk I have already 

 hinted, when speaking of the Faxoe limestone, p. 316, and it was 

 also remarked in an early part of this volume, that even some of that chalk, 

 which appears to an ordinary observer quite destitute of organic re- 

 mains, is nevertheless, when seen under the microscope, full of frag- 

 ments of corals, bryozoa, and sponges ; together with the valves of 

 entomostraca, the shells of foraminifera, and still more minute infu- 

 soria, (See p. 26). 



Now it had been often suspected, before these discoveries, that white 

 chalk might be of animal origin, even where every trace of organic 

 structure had vanished. This bold idea was partly founded on the 

 fact, that the chalk consisted of carbonate of lime, such as would 

 result from the decomposition of testacea, echini, and corals; and 

 partly on the passage observable between these fossils when half de- 

 composed and chalk. But this conjecture seemed to many naturalists 



