1880.] 369 [Crosby. 
admitting that the rock has been powerfully compressed in the 
right direction to produce the most at least of the indentations, or, 
perhaps, all, if we make allowance for, the necessarily irregular action 
of the pressure in a conglomerate. 
In other words, the pressure has been exerted, and exerted in the 
right direction to produce the effect. That this is a mere coinci- 
dence is possible; but before we adopt that conclusion, and reject a 
force proved to exist and at once so simple and so efficient, it seems 
only reasonable to make sure that the counter-explanation is at least 
equally satisfactory. But we can go further than this; the argu- 
ment for the compression theory has not been fully stated. We 
are not only able to prove without reference to the indentations that 
the pebbles have been powerfully compressed in the right direction 
to produce these marks, but we are also in no wise dependent “upon 
the indentations for evidence that some at least of the pebbles have 
been truly plastic ; for a certain degree of plasticity must be admitted 
in those cases, and these are by no means rare, in which fractures 
have produced fissures of sensible width (one to six mm.) on one 
side of the pebbles and yet failed to break them in two, the fractures 
ending before reaching the other side. I have frequently observed 
these gaping and yet incomplete fractures on surfaces recently 
exposed by quarrying, or by my own efforts with the hammer, 
where it was impossible that they should have been widened by 
the action of the elements. Here, then, we have evidence en- 
tirely independent of the indentations, and apparently of an irref- 
ragable character, proving that the pebbles have been com- 
pressed in the right direction to produce the indentations, and that 
they have been plastic. In my estimation, the only important point 
not readily explained by the compression theory, is the elongation of 
many of the indentations; and even this is partially, perhaps 
entirely, accounted for by the squeezing-out tendency,— the exten- 
sion of the material of the rock in the plane of bedding,— of which 
the cleavage, fractures, and polar cavities afford abundant evidence. 
Although the indentations, as they appear on the surface of the 
ledge, are found mainly upon the upper northern aspect of the 
pebbles, yet I must insist that the east-west trend of the most of 
those which are sensibly elongated, is an insuperable objection to the 
glacial theory of their origin. And still less competent is this 
theory to explain those impressions which are found, as I certainly 
PROCEEDINGS B. 8. N. H. — VOL. XX. 25 AUGUST, 1880. 
