240 ANIMAL OKIGLST OF WHITE CHALK. [Ch. XVIL 



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 de- 

 posits of recent date. Pure chalk, of nearly uniform aspect and compo- 

 sition, is met with in a northwest and southeast direction, from the north 

 of Ireland to the Crimea, a distance of about 1140 geographical miles, 

 and in an opposite direction it extends from the south of Sweden to the 

 south of Bourdeaux, a distance of about 840 geographical miles. In 

 Southern Russia, according to Sir R. Murchison, it is sometimes 600 feet 

 thick, and retains the same mineral character as in France and England, 

 with the same fossils, including Inoceramus Cuvieri, Belemnites mucro- 

 natus, 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 Radack, with sev- 

 eral 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 may be scattered far and wide by oceanic 

 currents. That this mud would resemble chalk I have already hinted 

 when speaking of the Faxoe limestone, p. 238, 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 remains, 

 is nevertheless, when seen under the microscope, full of fragments of 

 corals, bryozoa, and sponges ; together with the valves of entomo- 

 straca, the shells of foraminifera, and still more minute infusoria. (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 struc- 

 ture has 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 decomposed and chalk. But 

 this conjecture seemed to many naturalists quite vague and visionary, 

 until its probability was strengthened by new evidence brought to light 

 by modern geologists. 



We learn from Capt. Nelson, that, in the Bermuda Islands, and in 

 the Bahamas, there are many basins or lagoons almost surrounded and 

 inclosed by reefs of coral. At the bottom of these lagoons a soft white 

 calcareous mud is formed, not merely from the comminution of corallines 

 (or calcareous plants) and corals, together with the exuviae of foraminifera, 



