Crosby.] 312 [November 5, 
yard, the broad, shallow grooves nearly all lying in the plane of 
the bedding and trending east-west. 
So far as the impressed pebbles are concerned, the evidence is 
plain that the compressing force has acted mainly in a direction nor- _ 
mal to the surface of the beds; and this is precisely the direction re- — 
quired by the cleavage of the slaty layers. 
Fractured pebbles are numerous in some parts of the ledge, and as 
a rule the fragments have been drawn asunder one to five mm. by a 
stretching or squeezing-out force ; while among both the fractured 
and unfractured pebbles there are abundant examples where the 
paste has separated from the ends or poles of the pebbles in obe- 
dience to the same tendency.. The fractures and cavities are fre- 
_ quently transverse to the strike of the rock, though sometimes coin- 
ciding with it; but they all agree in being at right angles to the bed- 
ding of the conglomerate. In other words, the fractures, cavities 
and indentations of the pebbles, and the cleavage of the slaty layers, 
in this ledge, all require compression in a direction perpendicular to 
the stratification; and this, I think, goes far to prove that the pecu- 
liar concave surfaces, or indentations, of the pebbles, are really the 
effect of pressure, and indicate that the now rigid quartzite has been 
formerly somewhat plastic. 
I have already stated that the elongation of many of the indented 
surfaces of the pebbles seems, if we accept the theory of their former 
plasticity, to require us to suppose that the pebbles have slipped over 
each other in the rock. But when we consider the fracturing of the 
pebbles, and the cavities formed at their ends and between their 
linear elements or fragments, it is impossible to deny that the rock 
has suffered an appreciable extension in the plane of the bedding; 
and therefore the relative movement or slipping of the pebbles in a 
definite direction must be accepted as an established fact. 
The elongated impressions can not be ascribed to glacial action, 
either before or after the deposition of the pebbles, on account of the 
variety of positions in which they are found and their nearly con- 
stant east-west trend. And if it were possible to conceive of any 
mode of attrition by which these forms could be developed, such an 
hypothesis would be equally negatived by the uniform direction of 
the grooves. That these indentations have been produced since the 
pebbles reached their present positions in the rock is unquestionable; 
and they must be the result either of pressure or of fracture. If we 
regard them as simply conchoidal fractures, we shall still find it im- 
