560 DE. J. W. EVANS ON MECHANICALLY-FOKMED [Aug. IQOO,- 



deposit that has already accumulated. Current-phenomena such 

 as false-bedding cannot, therefore, be expected to occur in calcareous 

 rocks formed in water under normal conditions. 



But although the water of seas and lakes is in general imperfectly 

 saturated with carbonate of lime, this is by no means always the 

 case. Under certain circumstances — especially in hot countries and 

 in the immediate vicinity of coral-reefs and other calcareous rocks 

 or deposits — the water becomes so loaded with calcareous material 

 that it will, instead of dissolving fine calcareous particles, cover 

 them with an envelope of carbonate of lime. If the water is not 

 in motion, adjoining particles will be cemented together into a 

 solid mass ; but if there is sufficient current or wave-motion to 

 prevent this, a free rounded grain consisting of a calcareous 

 envelope surrounding the original object will be the result. If the 

 saturation and movement of the water continue, the process will 

 be again and again repeated till the grain is too large to be 

 disturbed by the motion of the water, and an oolite-grain will 

 be formed. 1 Nor is there any necessity that the nucleus should 

 be calcareous : a fragment of quartz, a crystal of hornblende, a 

 siliceous sponge-spicule, or perhaps even a particle of organic 

 matter, will serve the same purpose." It will be seen at once that 

 the conditions under which oolite is formed — rapid movement and 

 the saturation of the water — are exactly those which we might 

 expect to favour the development of current-phenomena in fine- 

 grained calcareous rocks, and we do in fact find false-bedding 

 commonly occurring in the typical oolites of the Jurassic period. 3 



Although calcareous rock formed by the mechanical action of 

 moving water presents some abnormalities as compared with 

 arenaceous deposits, the subaerial calcareous beds accumulated by 

 the transporting power of the wind have the closest resemblance 

 in structure to acolian deposits formed of other materials. The 

 usual sources of the grains of a wind-formed calcareous rock are the 

 littoral deposits on the sen -shore, though upraised subaqueous 

 deposits and earlier seolian accumulations may contribute their 

 share. Calcareous particles of marine origin — foraminifera and 

 other minute organisms, small fragments of shell or coral, and' 



1 See [49] p. IOC), [25] p. 48:j. [46] pp. 14, 10 & [52] p. 102. It does not fall 

 within the province of this paper to discuss the action of algje in promoting 

 the formation of oolite-grains. Nor has it yet been proved that their presence 

 is a necessary condition ; but it is possible that in some cases they cause the 

 formation of oolite, even in unsaturated water. [Numbers in brackets through- 

 out this paper refer to the Bibliography, § X, p. 581.] 



2 Darwin described the deposition of carbonate of lime at Ascension on 

 a large rock-surface when the sea-water was saturated with carbonate of lime 

 (in the hot season), and its disappearance by solution or abrasion when (in the 

 cold season) the water was again in an unsaturated condition : see [14] p. 50 ; 

 [27] A p. 57 & B pp. 198-99. He also recorded at St. Helena the covering 

 of pebbles with a calcareous pellicle by the action of percolating water, and 

 apparently the formation of oolite in the same manner, [14] pp. 86-89 ; [27] 

 A pp. 88-101 & B pp. 222-24. 



[45] p. 282. We must not, however, lose sight of the possibility that some 

 of these Jurassic oolites may be of icoliau origin : see p. 579. 



