FORMATION OP OOLITE. 19 
in the Bermudas and Bahamas of thick Serpuline reel's, made up 
largely of Scrpulat ; and of such layers we have examples in the 
Inferior Oolite, and in the Portland Beds. 
The formation of oolite has been discussed by Mr. Hudleston, 
who, basing his conclusions on the observations of Dana, remarks 
that oolite is for the most part granulated coral mud* Thus the 
fine calcareous mud, or chalky silt, that is worn from the reefs may 
be carried away great distances and deposited as ordinary sedi- 
ment. Nearer to the reefs the matter that subsides " encounters 
an acid stratum of water, due to the quantity of carbonic acid 
generated by the decomposition of organic matter and the respira- 
tion of animals." The same acid is also conveyed to sea water 
from the atmosphere by rain. By the aid of this acid the soluble 
bicarbonate of Jime is formed from the calcareous sediment as it 
subsides, but in its downward passage the excess of carbonic acid 
is no longer found, and the calcic carbonate is precipitated 
amongst the interspaces or on particles of the slowly settling mud. 
Sorby stated his opinion that the normal oolitic grains " indi- 
cate the original deposition of calcite round nuclei gently drifted 
along by currents of the ordinary temperature." f It would thus 
seem that the grains were formed in agitated water in which were 
present minute particles of quartz, Foraminifera, and fragments of 
other organic remains ; and that the water was charged with much 
carbonate of lime in solution. The mechanical impurities acted 
as nuclei around which the carbonate of lime was deposited, while 
the grains were kept in motion, so that all sides would be 
encrusted. (See also p. 14.) 
This explanation is not inconsistent with that applied long ago by 
Werner to the " Sprudelstein," of Carlsbad. There " particles of sand 
are raised in the water by means of air-bubbles, and become covered with 
calcareous earth, which is deposited around them in lamellar concre- 
tions."]; When the particles become too heavy to be thus supported they 
subside. The opinion has also been expressed that thin films of carbonate 
of lime have sometimes been formed around the air-bubbles, from which 
the globules of oolite have been formed by further accretions. 
Among the Oolitic rocks that have been examined under the 
microscope, there are some that show granules with but a thirt 
coating of calcareous matter. Where the nuclei were very heavy 
they were rolled over and plastered with layers of mud and 
Girvanclla, as in the case of the Pisolites. 
Other grains of oolite come closely in contact and appear to be 
welded together and partially indented. It is remarkable that 
these features are represented in calculi, which show both the 
radial and concentric structure of ordinary colite grains. 
* Proc. Geol. Assoc., vol. v. p. 431, and Gasteropoda of Inf. Oolite, p. 22. See 
also Phillips, Geol. Oxford, p. 396 ; Wright, Proc. Cotteswold Club, vol. iv. p. 97 ; 
vi. p. 136 ; and Fox-Strangways, Jurassic Rocks of Yorkshire, vol. i. pp. 398, &c. 
f Address to Geol. Soc. 1879, Quart. Journ. Geol. Soc., vol. xxxv. (Proc.), p. 75. 
j Jameson's Mineralogy, ed. 3, vol. ii. p. 540. 
See Figs. 3 and 4, pp. 11 and 12, of G. Rainey's Mode of Formation of Shells 
of Animals, of Bone, and of several other structures. 8vo. London, 1858. See also 
Sorby, Address to Geol. Soc., 1879 ; reprint with plates, Plate XVI., fig. 1. 
B 2 
