122 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
which may afford as high as 73 % to 79 % of silica. The various 
mineral compounds capable of forming from the percentages of 
elements present in different portions of the magma are found in 
the rock types of the several localities, and from a series of gross 
analyses of the rock types, such as we hope may some day be 
afforded, these component minerals may be computed. Rarely is an 
area found where a better opportunity is afforded for the chemical 
study of magmatic differentiation than that which the petrographic 
study of the Blue Hills area suggests. 
The field relations of the various batholitic zones composed of the 
rocks described in this paper have been carefully worked out by 
Professor Crosby, and hence receive no further discussion here. 1 
SEDIMENTARY ROCKS. 
Slates and Limestone.— The Cambrian slate of the Hayward 
Creek Paradoxides locality 2 is of a greenish gray color, somewhat 
siliceous, heavily bedded, and uniformly fine-grained. Grains of 
pyrite are diffused through it, as well as through the dikes that 
pierce it. Microscopically it is of very fine texture and mottled in 
color; the mottlings being grouped about particles of menaccanite. 
Discoloration by iron oxides is frequent. 
The slates of Weymouth Fore River and Mill Cove are banded 
with thin purplish or chocolate streaks and contain nodules or 
cavities of irregular forms and sizes, usually conforming to the 
direction of the bedding planes. These cavities are coated or 
filled with alternate yellow or greenish, and dark green or brown 
layers; frequently, in the larger ones with earthy matter in the 
center (PL 2 Fig. 9). They seem to be due to alteration, near the 
granite contact, of original calcareous segregations or claystones in 
the slate, before litliification of the slate. Further away from the 
Simple Petrosilex -< 
1 S Godon (’09) classed the rhyolites as 
Flinty Petrosilex 
Sonorous “ 
Jasper “ 
__ Novacular “ 
Porphyritic Petrosilex.” 
He remarks upon “ the transition of feldsparoid to petrosilex and porphyritic 
petrosilex ” in the Blue Hills. See also Crosby (’80, p. 91) and Hitchcock (’41, p. 605.) 
2 See Rogers (’56), Ordway (’61), and Hunt (’72). 
I 
