G. C. Crick—Two Type-specimens of Ammonites. 503 
beneath the loaded area, and inwards towards the area relieved of its 
load. He rightly uses the word plastic here. And here is another 
contrast between the properties of the ice-sheets which played an 
important part in the formation of some Canadian deposits and the 
rocks themselves. The ice moved much as a truly viscous liquid 
would have done whilst the deep-seated rocks underwent plastic 
deformation. The ice formed and maintained an almost level upper 
surface like that of the Antarctic Plateau, whilst the plastic rocks 
allowed themselves to be cut into hill and dale until the stresses 
produced were sufficiently great to allow of plastic flow. Plastic 
solids never reach a condition in which the stresses are equal in all 
directions ; they cease to flow when the stress falls below some particular 
value. Although it is true that the solid, liquid, and gaseous states 
are continuous, the liquid differs from the plastic solid inasmuch as it 
yields to any stress however small, whereas the solid does not begin to 
deform continuously until the unbalanced stress reaches some particular 
value depending upon the nature of the material and the temperature. 
With rise of temperature this limiting stress becomes smaller and 
smaller, and when it falls to zero the solid becomes a liquid. For 
all practical purposes, however, substances which flow (shear) under 
very small stresses may be regarded as viscous (liquids). Professor 
Coleman shows that although the ‘ Canadian shield’ was depressed by 
the weight of the Pleistocene ice resting upon it, it has not risen as 
much as might have been expected considering the weight of ice 
melted away; probably this is also owing to the plastic nature of the 
earth’s crust. 
There is another interesting property of plastic substances which 
might be referred to. When a test bar of iron or steel is drawn out 
to some extent in a testing machine and its length then measured, it 
will be found that, after giving it a light blow, it will have shortened 
again somewhat. If the bar has suffered compression it will have 
lengthened again. In many cases it has been found that an earth- 
quake has resulted in a rise or fall of the land locally. May it be 
that the jar of the strained plastic crust has released strains in 
a similar manner and given rise to sudden change of form ? 
VI.—Nore on tHE TYPE-SPECIMENS OF AWMONITES CORDATUS AND 
AMMONITES EXCAVATUS, J. SOWERBY. 
By G. C. Crick, F.G.S., British Museum (Natural History).! 
SOWERBY’S type-specimens of Ammonites cordatus and 
» Ammonites excavatus have been dealt with comparatively 
recently by Miss M. Healey in the Palcontologia universalis (sér. 11, 
fasc. 1, August, 1905, Nos. 94, 94a, and 92, 92a, 926 respectively), 
but her statements require a little modification in some details. Both 
specimens are in the British Museum collection, and were described 
and figured in the Iineral Conchology. 
1. Ammonites cordatus.—Miss Healey gives the date of publication 
of this species as 1812, and observes that ‘‘ The date 1813 is given on 
the plate, but on the title page the date is 1812”. From this remark 
1 Published by permission of the Trustees of the British Museum. 
