42 SEASONAL DEPOSITION IN AQUEO-GLACIAL SEDIMENTS. 
5. THE ROXBURY CONGLOMERATE SERIES 
According to the work of La Forge, the Roxbury conglomerate series is 
described in part as follows: 
“The lower formation, named for the Roxbury district of Boston, where it is conspicu- 
ously exposed, consists of a thick conglomerate and some sandstone and slate. In at least 
the southern part of the basin it may be divided into three members — the Brookline con- 
glomerate at the base, the Dorchester slate in the middle, and the Squantum tillite at the top. 
The later flows of the Mattapan volcanic complex, chiefly amygdaloidal melaphyre, are at 
several places interstratified with the Brookline and Dorchester members, but they are not 
known to occur in the Squantum member. It is impossible to distinguish everywhere be- 
tween some of the earlier beds of the Brookline member and some of the volcanic conglom- 
erates of the Mattapan complex, but clearly the volcanic activity began before the deposition 
of the Brookline and it appears to have ceased, at least in so far as surface extrusion is con- 
cerned, before the advent of the glacial conditions that marked the close of Roxbury time. 
“ Brookline conglomerate member.— This member is named from Brookline, where the 
rocks are extensively exposed. It. consists of massive conglomerate from 500 to perhaps 
2,000 feet thick, which contains some layers or pockets of sandstone and a few thin lenses of 
slate. At some places along the southern margin of the basin its base is a slaty or sericitic 
quartzite but at most places it is a coarse ill-sorted conglomerate containing some pebbles 
or small bowlders more than a foot through. 
“ Dorchester slate member.— This member is named from the Dorchester district of Boston, 
where it is exposed at several places. It consists of red and purple slates, in part cross- 
bedded, interbedded with sandstone and fine-pebble conglomerate. The slate is typically 
rather coarse grained and consists largely of reworked volcanic sediments. In Dorchester 
and in the southern part of the basin generally the member is 100 to 600 feet thick, but if the 
slate exposed in and about Allston Heights is assigned to the Dorchester member, its maxi- 
mum thickness may be as much as 1,000 feet.” Emmrson, 1917, p. 56, 57. 
THe Squantum TiLtuir—e Memper. The exact sequence of the tillite beds 
and associated deposits at Squantum is difficult to prove for several reasons. 
First, the more or less continuous outcrops exhibiting few different horizons of 
the formation, strike along the shore, on the north of Squantum Head, and 
southward from these exposures except in the old quarry in the top of the hill, 
glacial drift covers the land. Secondly, several faults obscure the sequence, 
or at least make it somewhat uncertain. From the fact that practically the 
same sequence can be made out at the Atlantic exposure, however, I have felt 
fairly sure that the sequence published in a former paper is correct (Sayles, 
1914, p. 155). The thickness of the tillite beds, however, was given incorrectly 
as 600 feet. Although the beds here are not doubled by folding the total thick- 
ness is probably not over 253 feet. Taking out the intercalated beds of con- - 
glomerate, sandstone, and slate, which were all included in the tillite formation 
