Wadsworth.] 316 [November 5, 
chemical analysis is able to prove that a mineral agerevate is a simple 
mineral (pinite). 
Forms that are more curved and irregular, elongated and distorted, 
than any that I have seen in the conglomerates, have been picked up 
by me on the sea beach and preserved for inspection at the Museum — 
of Comparative Zoology. That these forms are products of frac- 
turing and wearing is shown by the absence of any signs of twist- 
ing of the laminae and by the stratification (in the stratified speci- 
mens) being continuous at both ends and worn across in the centre. 
The lamination and stratification do not correspond with the con- 
tours of the pebbles. Aitention is called to this, lest it should be 
claimed that the specimens had been bent in some conglomerate, 
which had since been broken up and its debris deposited on the shore. 
If one listens to the grinding and rubbing, when even a moderate 
swell strikes against the shingle he will wonder not at the amount of 
wear the pebbles show, but at the smallness of it, compared with that 
which one might expect. This is probably owing to the buoyant 
and cushioning effect of the water. The wear is, however, sufficient 
to give numerous singular forms which are directly proportional in 
number to the average force of the waves beating upon the shingle 
in question : rare fn sheltered spots, quite common in exposed ones. 
The pebbles from the conglomerate that Mr. Crosby has brought 
before us to-night were found on an elevation which was evidently of 
similar form during the glacial epoch. As this formed the crest 
and side of a little, exposed ridge, it was very powerfully abraded. 
The pebbles being of quartzite (¢ndurated sandstone) have been less 
affected by weathering than most of the pebbles of a different 
nature, and therefore retain much of the original glacial gouging, 
tearing and polish nearly intact. As the matrix is less resistant than 
the pebbles, the latter, besides having their tops planed off, were gen- 
erally more or less rounded on their sides. This rounding naturally 
took place where the matrix was most continuous and abundant, 2.e., 
parallel to the bedding planes. Where the surfaces have been ex- 
posed to weathering since, the matrix has decomposed, setting free 
the coarse quartz sand, of which it is largely made up. Certain of the 
markings upon the pebbles that have been ascribed to indentation 
have probably been produced by the churning of this sand by the 
rain against their sides, and by the wear of the sand carried in the 
little streams produced by the rain. The matrix is so closely adher- 
ent to the sides of the pebbles, that oftentimes the pebble will be 
