8 
the main the medium texture of the Roxbury puddingstone, the 
pebbles not exceeding four inches in diameter; but at some 
points, such as Great Hill, Green Hill Ledge and the small 
area north of Crescent Hill, they are in part much larger, one 
to two feet. The larger pebbles are chiefly granite and mela- 
phyr, the felsite having been always, as now, too brittle and 
too finely jointed to form many large pebbles. The conglom- 
erate is usually a typical puddingstone, presenting distinctly 
the character of a breccia only where it immediately overlies the 
granite, and is largely composed of local granitic débris. 
It is very noteworthy that the quartzite pebbles, which are 
such a prominent feature of the conglomerate in most parts of 
the Boston Basin, are almost entirely wanting in the Nantasket 
ledges ; indicating that formerly, as at the present time, the 
ancient quartzites had but a slight development on this side of 
the basin. The same re: soning will not, however, explain the 
general absence of pebbles of diorite in the conglomerate. 
Next to granite, diorite is by far the most abundant’of the older 
rocks bordering the Boston Basin, covering an aggregate area 
of at least one hundred square miles within ten miles of the pres- 
ent margin of the basin. Since the diorite is every where inter- 
sected by, and therefore older than, the granites, it is unques- 
tionably also older than the conglomerate ; and we are obliged 
to suppose that, like the other of our older rocks, it was proba- 
bly abundantly exposed over the surface of this region during 
the formation of the conglomerate. A few pebbles in the con- 
glomerate, in different parts of the basin, may be doubtfully 
referred to the diorite; but, generally speaking, this rock has 
contributed little or nothing to the composition of the conglom- 
erate. Bowlders and finer débris of the diorite are extremely 
abundant in the glacial drift of this section ; but it is noticeable 
that the smaller masses, especially, are usually more or less 
decomposed, showing much more alteration than similar frag- 
ments of granite or felsite. If, therefore, as already suggested, 
we may follow Mr. Douvé! in attributing the formation of the 
early sediments of this region to the working over by the sea of 
Proc. B. S. N. H., XXIII., 20-80, 
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