274 Proceedings of the Royal Physical Society. 
by its fossils. I would refer to the relation of the Moffat 
Terrane, with its graptolite zones, to their chronological equi- 
valents in North Wales; or to the Lias, with its Ammonite 
zones, in illustration of what is meant here. If possible in 
making comparisons, sedimentary rocks of one petrographical 
ty pe—such as there is reason to believe must have been formed 
at nearly the same rate through all geological periods—should 
be uniformly employed. Marine limestones are certainly the 
best for this purpose, except where, as in the case of some 
few rocks formed since Eocene times, these may happen to 
consist of true coral-reefs. The Chalk appears to me to be 
a typical deposit of this kind, as it was certainly formed in 
deep water, and at a rate sufficiently slow to admit of its 
including records of a long succession of changes of life— 
longer, I should think, than the whole of the period that has 
elapsed since its formation ceased on this part of the earth’s 
surface. 
In the neighbourhood of Southampton, and again near 
Norwich, the Chalk exceeds 1100 feet in thickness. On the 
east side of the North Sea it is even thicker than that. 
From bottom to top, it is almost entirely free from admixture 
with rock material of purely terrigenous origin, and its 
general petrographical character is that of a deposit formed 
under much the same conditions as the foraminiferal oozes of 
the ocean of the present day. 
If we assume, as was done in the case of the Nummulitic 
Limestone, that 1 foot in 25,000 years fairly represents the 
rate at which limestones are now being (and have always 
been) formed, then, not taking into account any interruption 
of deposit, a thickness of 1100 feet of Chalk requires 
27,500,000 years. 
If this is the case, we need hardly marvel at the gradual 
disappearance of the Cephalopoda of the Chalk, nor at that of 
the great reptiles. According to the view here set forth, the 
Ammonites and Belemnites died out almost entirely within 
the Cretaceous Period itself. Only one Ammonite is known to 
survive to the period represented by the topmost bed of the 
Chalk, and the great reptiles appear, one by one, to have 
vanished from the face of the earth at an earlier period even 
