1892.] 
[Crosby. 
Si i 
the drift formations and the sea, — cut off, alike from the strati- 
fied rocks of Nantasket on the east and those of Weymouth 
and the Blue Hills on the west. Notwithstanding these diffi- 
culties, however, we have two clues which are satisfactory so far 
as they go, although they are, unfortunately, not of such a 
nature as to lead to a definite determination of the geological 
horizon. These are (1) the relations to the older eruptives — 
the granitic rocks ; (2) the composition of the conglomerate. 
The Paradoxides beds of Braintree and the Blue Hills, which 
Walcott now regards as of Middle Cambrian age, are clearly 
intersected by, and therefore older than, the different varieties 
of granite and felsite of that district. We have no reason to 
doubt that these granitic rocks are the same for the entire South 
Shore, from Scituate and Cohasset westward ; and therefore it fol- 
lows that the conglomerates of Nantasket and Ilingham, which 
are so largely composed of the debris of these eruptives and are 
seen in several sections to rest directly upon them, must repre- 
sent a horizon above the Paradoxides beds. The conglomerate 
series is overlain conformably by the great slate series of Hing- 
ham, with some interstratification or blending of the two series. 
We are thus obliged to recognize in the Boston Basin a thousand 
feet or more of argillaceous strata above the Paradoxides or Mid- 
dle Cambrian zone and separated from it by a corresponding or 
greater thickness of coarse sediments and lavas — the conglomer- 
ate series, with a probable unconformity at the base of the lat- 
ter. This unconformity between the Paradoxides beds and the 
conglomerate series is not only proved by the extensive erosion 
of the granitic rocks, but we also find in the conglomerate, at 
Huit’s Cove and elsewhere, pebbles of slate similar to that of the 
Paradoxides beds. Of special interest in this connection, as 
already explained, are the pebbles of limestone in the conglom- 
erate of Huit’s Cove. Limestone is a rare rock in Eastern Mas- 
sachusetts ; and the only beds now known that can be regarded as 
a probable source of these pebbles are the limited and impure 
layers in the Cambrian slates atNahant and Weymouth, and pos- 
sibly at Stoneham and other points outside of the Boston Basin. 
It is apparent from the foregoing that, although we may fairly 
regard the stratified rocks of Hingham as forming one conform- 
able series from the lowest conglomerate to the highest slate, and 
although this series, which is quite certainly 2000 and probably, 
