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Proceedings of the Koyal Society of Edinburgh. [Sess. 
and pressure of the superincumbent basalt. Though the effect of this 
change has been to convert them either wholly or partially into crystalline 
silica, and in some of them to obliterate to a great extent the micro- 
organisms they contain, yet when these can be made out, usually in outline 
as in many flints, the shell fragments, Foraminifera, and sponge spicules 
preserve their natural form without sign of distortion. It will be seen 
from the evidence given in the preceding pages that there appears to be 
a passage from a terrigenous to a partly calcareous deposit, and in this 
passage there are stages marked by the abundance of Inoceramus prisms 
and sponge spicules, culminating in an ooze consisting of the debris of 
calcareous organisms and Foraminifera. The traces of sponge spicules 
and colloid silica in globular form in many of the sections of silicifled 
chalk from Argyllshire and Mull, and in the fragments contained in the 
white sandstone of Eigg, are evidence that spongarian remains occurred in 
abundance and that a portion of the strata at least may be regarded as 
sponge beds. 
I regret that I cannot say with absolute certainty that Radiolaria 
exist in these siliceous chalks. I am, however, convinced they are present, 
and when sufficient material has been examined I believe they will be 
ultimately identifled. 
If my reading of the succession is correct, the change from a terrigenous 
deposit to a calcareous ooze is rapid and possibly indicates current action 
which probably prevented the accumulation of sediment. The glauconite 
base of the siliceous chalk would support this theory, and indeed current 
action is not improbable when we reflect that the siliceous chalk succeeds 
sandstone believed to be of estuarine origin. 
In the absence of fossil evidence it would be mere speculation to 
attempt to correlate the siliceous chalk of the West of Scotland with those 
they most resemble at Belhelvie. It does not follow that because sponge 
beds and Inoceramus chalk are represented that they are necessarily on the 
same horizon ; they can only be regarded as similar phases of sedimentation 
which are likely to occur not far from the coast of a land undergoing 
gradual submergence. 
It is possible that the exposures of the Cretaceous strata in the West 
of Scotland do not give us the true succession — Professor Judd remarks, 
in referring to the chalk exposed behind Carsaig House, “ At two separate 
points I have detected masses of the altered chalk- and flint-beds squeezed 
out from beneath the mass of superincumbent basalt.” ^ 
* “The Secondary Rocks of Scotland,” Q.J.G.S., vol. xxxiv, 1878, p. 730. 
(Issued separately December 14, 1915.) 
