1879.] Sk Crosby. 
weathered surfaces of the conglomerate. I have satisfied myself, 
however, that they really exist on all sides of the pebbles, the action 
of the weather being simply to remove the cement and expose the 
true forms of the pebbles. The conglomerate is so firm that a peb- 
ble dislodged by the hammer usually carries with it enough of the 
cement, or adjacent pebbles, to obscure all but the deepest impres- 
sions. 
These indentations or dimples present some very curious features. 
In the quartzite pebbles especially, the surfaces which have yielded 
are remarkably smooth, smoother than an ordinary weathered sur- 
face, — smoother even than a glaciated surface; in fact the com- 
pression appears to have compacted and polished the material, the 
pebbles being case-hardened as it were, at the points where the pres- 
sure was applied. 
A large majority of the impressions are very long in proportion to 
their breadth; appearing as shallow grooves on the surfaces of the 
pebbles, and reminding me of finger-marks on a ball of putty. I 
consider this elongation of the indented surfaces as their most puz- 
zling characteristic, and no entirely satisfactory explanation has yet 
occurred to my mind. Itseems necessary to suppose that the pebbles 
have slipped over each other in the rock. The longest grooves are 
more than three inches long, and at least four times longer than broad. 
They sometimes extend on the surfaces of the pebbles through an are 
of nearly ninety degrees; and occasionally the same groove is con- 
tinuous across two or three small pebbles. The elongated impres- 
sions appear to pass gradually into those of more normal form; and 
must, I think, be explained in the same way. 
Where two impressions lie near together on the same pebble, the 
tendency to squeeze up the material between them to an acute angle 
is sometimes very evident. Although occurring on all sides of the 
pebbles as they lie in the rock, the indentations are found princi- 
pally on the two opposite sides which coincide with the bedding; the 
line connecting the indented surfaces of a pebble usually passing 
through its centre and in a direction perpendicular to the strata. 
This constancy of position is especially characteristic of the elon- 
gated impressions, the great majority of these being found on the 
upper northern aspect of the pebbles; and their direction is equally 
constant, being almost invariably east-west, or parallel with the 
strike of the conglomerate. There are places on the ledge where 
thirty or more of the grooved pebbles may be counted in a square 
