300 Proceedings of the Royal Society of Edinburgh. [Sess. 
however, indications of sponge spicules. Numbers of small quartz grains 
are to be seen thickly scattered throughout the section, and the occurrence 
of opaque spots, blotches, or streaks seems to indicate that fine terrigenous 
matter occupied the interstices between the fragments. A large part of 
the finer material seems to have consisted of lath-shaped particles which 
have preserved this identity notwithstanding the alteration of the rock. 
Viewed as a whole, there is a curious resemblance in the section to the 
grey-green friable rock of Belhelvie, but there is no glauconite. 
These siliceous chalks everywhere overlie beds containing much detrital 
matter, whether it be the white sandstone or possibly in closer connection 
with the Greensand, as at Carsaig. As the period was one of submergence, 
it may be expected that the lower part of the siliceous chalk next above 
the sands would contain terrigenous matter, and that higher in the series 
the deposit would become gradually more calcareous. 
Though I have no accurate knowledge of the stratigraphical position 
occupied by the specimens from which the thin sections were obtained, 
yet when this series is reviewed together changes of structure due to 
different conditions of sedimentation are evident, and there seems to have 
been a rapid passage from a mud containing much terrigenous material 
to a comparatively pure calcareous ooze. 
Thus the white limestone of Carsaig (Mr Clough’s specimen) contains 
abundant evidence of terrigenous material in the quartz grains scattered 
throughout it in the lath-shaped particles in the finer part of the deposit. 
I cannot think that this was ever a chalk in the common acceptation of the 
term, but was mainly an inorganic deposit, though probably with a con- 
siderable admixture of calcareous material. In Professor Judd’s sections — 
O 4, Beinn-y-Hattan, and R 4, Gribun — quartz grains are scattered through 
the rock, fairly abundant in the former, less so in the latter. Though 
more obscure than in many examples, both the sections show traces of 
Foraminifera, shell fragments, and sponge spicules: in one part of the 
Gribun section spicules are exceptionally numerous. 
In O 3, Carsaig, and 0 4, Beinn-y-Hattan, which are described by Pro- 
fessor Rupert Jones as “ Glohigerina chalk,” sand grains are again present 
mixed with the shell fragments, Foraminifera, and spicules. Fine inorganic 
matter is not recognisable in these sections, unless a certain dirty streaky 
appearance is an indication of its presence. The remaining sections give 
us evidence of terrigenous material. 
O 2, Carsaig, and O 3*, Beinn-y-Hattan, are described by Professor 
Rupert Jones as “ Inoceramus chalk,” from the large number of Inoceramus 
prisms they contain. Professor Rupert Jones makes no mention of sponge 
