OOLITIC GRAINS. 11 
The phenomenon known as "lustre-mottling" is shown in some 
of the oolitic freestones of Barnack, Ketton, and Ancaster, and in 
the sandy limestone of Dene Mill, Great Weldon. It occurs also 
in calcareous grits associated with the Kimeridgian rocks in 
Sutherlandshire and near Cromarty. Attention was first called to 
such appearances in calcareous grit by Sedgwick, who referred to 
it as " chatoyant lustre."* 
These appearances are seen on a fractured surface of a rock 
when the lustre from the cleavage of a mineral is broken, and as 
it were " mottled," by the occurrence of inclusions. In the case of 
the oolites, the mottling is produced by the oolite grains that are 
included in the matrix of calcite ; and in calcareous sandstones the 
grains of quartz act in the same way. In the freestone associated 
with the Barnack Rag the oolite grains, showing concentric and 
radial structure, are embedded in a matrix of clear crystalline 
calcite. The individuals of calcite are of immense size, far more 
than sufficient to fill the field of view of a 1 inch object glass. 
The oolite grains lie in the crystalline individuals of the matrix, 
as do the grains of sand in the Fontainebleau Sandstone. 
Origin of Oolitic Grains. 
The first question of importance is whether the oolitic structure 
is original or secondary. I have seen no evidence in the slides 
to suggest that it is secondary. The occurrence of oolitic rocks 
without any matrix, and the presence of broken oolitic grains, are 
facts which point strongly to the conclusion that the structure is 
original. Assuming then that the structure is original we have 
to consider the causes which may have given rise to it. Rounded 
fragments of organic bodies are easily accounted for by the 
ordinary process of mechanical attrition. The origin of pisolitic 
grains is more complex ; Girvanella-growth and mechanical 
picking up of foreign bodies have both operated. An important 
question arises as to whether this picking up is purely a mechanical 
process or whether it may not have been facilitated by the simul- 
taneous precipitation of carbonate of lime from solution. In the 
case of the typical pisolitic grains it is not easy to answer this 
questiou. The oolitic grains showing concentric and radial struc- 
ture certainly appear to have grown by deposition of the kind 
referred to. Minute crystals and granules were deposited on 
the surfaces of the grains. Nevertheless it is probable that even 
in the case of these grains the growth was not solely due to this 
cause ; but that it was supplemented by the picking up of ex- 
traneous particles. If the grains had resulted solely from crystal- 
line deposition we should expect to find them more transparent 
and the individual grains of a simpler and more uniform character 
than is actually the case. 
As regards the amorphous pellets so common in many rocks 
and which frequently constitute the nuclei of oolitic grains ; 
* Trans. Geol. Soc., ser. 2, vol. iii. p. 4-64. 
