282 JURASSIC. 



]\Iammals, " There were giants in those days ! " but, descending to 

 earlier periods, we find only dwarfs.^ 



Among the fossils of the Oolitic strata INIollusca, however, are 

 the most abundant, and variously distributed, Cephalopods, 

 Gasteropods, Lamellibranchs, and Brachiopods being in turn 

 prevalent in different areas and at different horizons. Among 

 the Lamellibranchs Lima pcciiniforniis fproboscideaj, Ostrca gregaria, 

 and Gojiiomya v-scripta have a wide range. Among the Echino- 

 derms are Heuiicidai-is, Acrosalenia, Echinobrissus, Clypeiis, and 

 Pseudodiadema. Corals are not uncommon in places, and many 

 reef-building forms occur : some species are found in lenticular 

 coral-beds. It has been observed that although the character 

 of the sea-bottom varied greatly over the British area contempo- 

 raneously and successively, the general characters of a coral 

 sea constantly prevailed in the Oolitic period;^ and it has been 

 suggested that certain outlying masses of oolite derived their 

 present form not mainly from denudation, but from original 

 formation. Of course Coral-reefs may stand up in isolated masses 

 amid sedimentary deposits ; and if ultimately preserved, the structure 

 would suggest an unconformity or fault ; ^ but while there can 

 be no doubt that such reefs existed during Oolitic times, we have 

 no clear proof of their being preserved. 



Our Oolitic rocks, in fact, were to a large extent derived from the 

 waste of Coral-reefs, the Oolite itself, as Mr. W. H. Hudleston has 

 pointed out, being for the most part granulated coral-mud. The 

 well-known Coral-sand of the West Indies, while it resembles 

 oolite, is mainly composed of rolled fragments of coral. On 

 the other hand, the grains of oolite frequently exhibit an internal 

 radiate structure, and are made up of concentric rings of carbonate 

 of lime ; while the material of which they are chiefly composed 

 has been deposited chemically around nuclei, such as Foraminifera, 

 or minute fragments of Corals, Echinoderms, and Mollusca, or very 

 fine grains of quartzose sand. There are instances where the 

 oolitic structure seems to be independent of any nucleus, and 

 where it must have been produced by chemical changes in the soft 

 mud, or subsequently when the deposit had become solidified.* 

 Marly clay, as well as limestone, may be oolitic ; and sometimes the 

 entire mass of the rock is made up of oolitic grains, but then the 

 rock is usually false-bedded, indicating that the grains must have 

 been drifted and heaped up by currents in comparatively shallow 



^ In reference to the fossils of the Oolites, see the works of the Palaeonto- 

 graphical Society : — Reptilia and Mammalia, by Sir R. Owen ; Corals, by H. 

 Milne Edwards and J. Haime, with Supplements by Dr. P. M. Duncan ; Echino- 

 dermata, by Dr. T. Wright ; Brachiopoda, by Dr. T. Davidson ; Great Oolite 

 Mollusca, by Prof. J. Morris and Dr. Lycett ; Trigoniae, by Dr. Lycett, etc. ; see 

 also Pala:ontology of the Yorkshire Oolites, by \V. H. Hudleston, G. Mag. 

 iSSo, 1882, 1884, 1885. (See also references, ante, p. 254.) 



- P. M. Duncan, Q.J. xxvi. 62. 



3 See W. O. Crosby, G. Mag. 1879, p. 296. 



* J. Phillips, Geol. Oxford, etc, p. 396 ; H. C. Sorby, Address to Geol. Soc. 

 1879. 



