1880.] 371 [Crosby 
adjacent pebbles ; while the material between the indentations has 
‘been squeezed up to an acute angle. 
It is, no doubt, often easy to explain the concave surfaces and 
accurate fitting of the pebbles as merely accidental, as incidents 
of the sedimentary process; but in this particular example I think 
the indications of compression are plain enough to require at least 
the provisional acceptance of the theory that the pebbles have been 
plastic, especially since the pressure and plasticity have been proved 
by entirely independent evidence. 
But after all, what large element of improbability is there in this 
matter of the deformation of quartzite pebbles? Quartzite is a vari- 
ety of sandstone, and sandstone is consolidated sand; and there is no 
question but that among these rocks we have all degrees of consoli- 
dation from incoherent sand to the firmest quartzite. A very slight 
degree of pressure will distort a ball of sand, or even of some rocks 
called sandstone ; and where, in passing to more throughly consoli- 
dated varieties, are we to stop and say, ‘‘ beyond this point no com- 
pression is possible?’’? And in this connection it is important to 
remember that the pressure exerted during some geological opera- 
tions is very great. 
Mr. Wadsworth has asserted that the distortion of the pebbles 
would be attended by the development in them of a fluidal 
structure. I do not know that.he has proved by microscopical exam- 
ination that the fluidal structure does not exist in the indented peb- 
bles of this ledge ; but it seems to me that the statement that dis- 
torted pebbles necessarily possess this structure, involves an assump- 
tion which we have no right to make ; viz.: that the plasticity of the 
pebble means the plasticity of the constituent particles thereof. The 
plasticity of a ball of sand does not reside in the single grains, but is 
due to their imperfect cohesion ; and even in the firmest quartzite, it 
is fair to assume that the solid grains are stronger than the cement 
which unites them. Consequently, the cement is the first to yield, 
but when it has yielded, the unequal pressure is thereby removed 
from the grains, and distortion may be accomplished without the aid 
of sensible plasticity in the latter. 
It will be said, however, that we must not regard the distortion as 
a purely mechanical process. Very true; but the argument is not 
affected when chemical agents are admitted. These things we 
know: the cement of the quartzite is mainly silicious; and water, 
possibly alkaline, must have been present, and under the influence 
