OOLITIC STRUCTURE. 7 
usually of crystalline layers of carbonate of lime, mostly calcite 
but sometimes aragonite. These grains are often replaced by 
ferruginous compounds and rarely by silica. The grains though 
small are variable in size, and are usually spherical but sometimes 
oval ; and in other instances the shape bears relation to the nucleus 
around which the calcareous coating has been deposited. 
Many rocks however that present an oolitic appearance, prove on 
microscopic examination to be very largely composed of struc- 
tureless grains or pellets ; and some, like those of the Coral Sands of 
the West Indies, are composed largely of tiny rounded and spherical 
fragments, with or without a thin envelope of calcareous matter. 
Examination in the field, and 
_Jj with the aid of a hand magnify- 
ing glass, is insufficient to enable 
a discrimination to be made 
between the rocks chiefly formed 
of true oolitic grain?, and those 
more largely composed of pellets 
or rounded fragments of or- 
ganisms. Thus some of the 
Lower Purbeck beds are com- 
posed of granular limestone in 
which true oolitic grains form 
but a subordinate portion. 
In some instances we find; 
three or four oolitic grains in 
Microscopic Structure of Oolitic the midgt of a pellfctj ag in the , 
Limestone (after Sorby). Inferior Oolite of Sleaford, 
Magnified 30 Diameters. Compound Oolitic grains also 
occur, and occasionally we find mechanical additions amid the 
encircling oolitic growths, as in the Corallian limestone of Keevil. 
Some of the mud-pellets in the Great Oolite of Burford, as- 
noticed by Mr. Teall, look as if they had been slightly pressed 
together when in a soft state. (See p. 19.) 
Among the Jurassic rocks, oolitic grains occur in the Lias, 
Inferior Oolite, Great Oolite, Forest Marble, Corallian Beds, 
Portland Beds, and Purbeck Beds. 
In the Lower and Middle Lias they occur mainly in the iron- 
stone ; but, both in those formations and in the Inferior Oolite, 
we often find tiny spheroidal grains of oxide of iron (limonite), or 
" iron-shot " grains, that as a rule exhibit uo structure. In these 
cases, as in some of the Lower Lias limestones, it does not follow 
that the particles replaced were originally oolitic granules : they 
may have been pellets of limestone or rolled fragments of organic 
remains. 
In a yellow and blue-hearted oolite which I obtained from the 
Lincolnshire Limestone of Castle Bytham, Mr. A. B. Dick found 
the grains in the blue rock to be coated with a thin film of pyrites, 
and the grains in the exterior yellow portion to be coated with a 
film of limonite, evidently produced by the oxidatiyn of the 
pyrites. The division between the blue and yellow oolite was 
