MANSFIELD: ROXBURY CONGLOMERATE. 175 
that the coarse conglomerate occupies a stratigraphically lower position 
in the series than the alternating beds. This fact will be more clearly 
brought out when the structure is discussed. 
It must not, however, be assumed that the heavy conglomerate is 
uniformly coarse throughout the Newton-Brookline area. ‘The oc- 
currence of boulders as large as two feet in diameter is rather local. 
There are, indeed, many ledges in the region where boulders of this 
size occur, but often in close proximity to them conglomerates of much 
finer texture appear. Thus in the ledges west of Walnut Hill (Boston 
VI, A 12, B 12, C 12-13) the pebbles average only one and a half to 
three inches, while the coarse conglomerate above mentioned is only 
a few hundred yards away. 
Among the finer conglomerates a similar lack of uniformity is to be 
noted. In places where the pebbles average two or three inches in 5 
diameter pebbles six to ten inches in diameter not uncommonly apperf 
scattered among the smaller ones. Even where the rock is largely 
grit, scattered pebbles more than one foot in diameter may be seen. 
This is well exemplified at the large quarry south of the Arnold, Arbore- 
tum (Boston VI, M 16). Layers of coa se conglomeraté are not 
infrequently interbedded with sandston ¡ÓN grit apparently well up in 
the series, as in the remarkable outcrop on the north side of North 
Beacon Street in Brighton (Boston V, H 26), where pebbles of granite 
and other rocks more than one foot in diameter occur. ‘This ledge 
has given rise to some controversy because of the appearance of slate 
masses that resemble clastic material but are two feet or more in length 
and nearly a foot in width. It has been maintained on the one hand 
that the slate masses are pebbles and on the other that they are pockets 
of slaty material laid down during the deposition of the conglomerate. 
After a careful study of the ledge the writer is inclined to concur in 
the latter opinion, as regards the larger masses at least. It is difficult 
to see how slate shingle of that degree: of coarseness could escape | 
comminution by the large pebbles of granite and quartzite so abundant 
in the same deposit. The occurrence of contemporaneous grit among 
the pebbles indicates contemporaneous erosion during the deposition. 
Some of the features above described are shown in Plate 2. The 
conglomerate at Squantum described in the previous chapter affords 
another remarkable instance of irregularity of deposit (Plate 3). 
While therefore it is true that there seems to be a gradation in 
coarseness of material from the conglomerates of Brookline to the 
sandy and slaty sediments north, south, and east, it depends not so 
much upon the size of the individual masses, for coarse and fine 
