MANSFIELD: ROXBURY CONGLOMERATE. 197 
apparently part of the felsite period of eruption and felsite pebbles 
are found in the conglomerate. 
(d) It is inconceivable that so great a thickness of unfossiliferous 
Sediments could be formed well into Carboniferous times. 
(e) Indefinite forms, possibly organic, have been found in the 
Slates and these present Cambrian father than Carboniferous facies. 
(f) The whole thickness of slate exposed in Somerville is capped 
< overlaid by quartzite, — a feature unique in the Carboniferous of 
Yew England but common in the Cambrian formations. 
The argument for the Carboniferous age of the slates is thus out- 
lined by the same writer (ibid., p. 82-84) :— 
(a) The originally highly carbonaceous character of the slates. 
(b) The commonly accepted view of the structure. 
(c) Their lithological similarity to the slate interbedded with the 
conglomerate. 
(d) Their comparatively fresh, unaltered and unmetamorphosed 
Condition. The known Cambrian areas are affected by dynamic 
and contact metamorphism. 
(e) The similarity of the igneous rocks north and south of the basin 
points to their former continuity, the excavation of the basin, and the 
deposition of the included sediments. 
(f) The inclusion-bearing dike in the Mystic quarry contains frag- 
ments of andesitic breccia quite like the breccia outcropping in 
Malden. he latter are Postcambrian. 
(g) The long line of exposures parallel to the line of the crystalline 
boundary ada only two doubtful cases of any intrusive rocks which 
appear to belong with any of the igneous rocks of the escarpment. On 
both sides of the slate in Medford and in Brighton occur conglomer- 
ates containing pebbles of these igneous rocks and the contacts are 
probably sedimentary. i 
ae the strongest argument for the Cambrian age of the slates 
is their resemblance to the known Cambrian rocks of He region and 
their marked dissimilarity to the known Carboniferous rocks of neigh- 
oring areas, especially to the rocks of the Narragansett Basin. The 
intrusion of the slates by felsitic rocks, if definitely proved, would be 
a strong argument for the Precarboniferous age of the former, if such 
Intrusion can be shown to be definitely felsitic or granitic, And infre- 
quency of occurrence need not necessarily weigh against the argument. 
The base of the slates is not certainly ex xposed. poe en penetrat- 
ing only the lower portion of the slates may exist without having as 
yet been discovered. Perhaps some of the numerous quartz veins 
