214 



LYELL'S ELEMENTS OP GEOLOGY. 



Oolite Group Fossil Corals. 



remains ; and though varying in thickness, may be traced in 

 certain directions for great distances, especially if we compare 

 the part of England to which the above-mentioned type refers 

 with the north-west of France, and the Jura mountains, which 

 separate that country from Switzerland, and in which, though 

 distant above 400 geographical miles, the analogy to the English 

 type above mentioned is more perfect than in Yorkshire or Nor- 

 mandy. 



To enter upon a systematic description of this complicated 

 series of strata would require many chapters ; the following 

 facts, tlierefore, are selected from a multitude of others, with a 

 view of illustrating the origin of the oolitic rocks, and of show- 

 ing the state of organic lile and geographical condition of part 

 of the globe when they were formed. 



In almost all the minor divisions enumerated in the above 

 Table, Ammonites and Belemnites are found, (see Figs. '218. 

 215.) but of species different from those of the cretaceous period. 

 The ammonites are of various sizes, from the size of a small 

 carriage- wheel to less than. an inch diameter. 



It is not uncommon to find belemnites in different members of 

 the series, with full-grown serpulsB attached to them. As these 

 shells, like the bone of the cuttle-fish, so often thrown on our 

 shores, were internal, it is clear, that after the death of the 

 cephalopod the belemnite remained for some time unburied at the 

 bottom of the sea, so that the serpulse grew upon it. 



These cephalopoda, swimming about in the open sea, left their 

 shells to be imbedded indifferently in whatever sediment was 

 then in the course of deposition, whether calcareous or argilla- 

 ceous. But the corals are almost entirely confined to the lime- 

 stones, and are wanting in the dense formations of interposed 

 clay, as also in the lias, these zoophytes requiring, not only car- 

 bonate of lime for their support, and clear water, but a bottom 

 remaining for years unchanged, either by the shifting of sand or 

 the accession of fresh sediment. 



In the Ui)per Oolite of England, corals are rare, although one 

 species is found plentifully at Tisbury, in Wiltshire, in the Port- 

 land sand, converted into flint and chert, the original calcareous 

 matter being replaced by silex. (Fig. 199.) One of the lime- 

 stones of the Middle Oolite has been called the " Coral Rag," 

 because it consists, in part, of continuous beds of petrified corals, 

 for the most part retaining the position in which they grew at the 

 bottom of the sea.- . They belong chiefly to the genera Caryo- 

 phyllia (Fig. 200.), Agaricia, and Astrea, and sometimes form 

 masses of coral fifteen feet thick. These coralline strata extend 

 through the calcareous hills of the N. W. of Berkshire, and 



