The age of the Porter Square camptonite has hOt been established with certainty.
The Porter Square camptonite was exposed briefly in 1981 during excavation for the Porter Square subway station in Cambridge, Massachusetts, within the Boston Basin.
Thin serpentinite and calcite veins cut the camptonite in some thin sections.
Some individual, rounded and embayed grains of albite are fractured with groundmass infilling fractures but in other instances feldspar aggregates appear to have separated slightly with gaps infilled by the camptonite groundmass (Fig.
In the Montreal area such rocks are mostly alkaline syenite, monzonite, basanite or alkali olivine basalt, and lamprophyre, especially camptonite (Hodgson, 1968; Eby, 1985).
The rock is probably camptonite, which commonly contains phenocrysts of an olivine group mineral.
Serpentine is a common alteration product of olivine in lamprophyric rocks such as camptonite.