A number of basaltic volcanic plugs are well exposed on the east flank of the Mt. Tom Range near Holyoke, Massachusetts. Flows, breccias, tuffs, sills, and dikes are associated with these centers and are partially interbedded with the Longmeadow Sandstone (Triassic) about 200 feet above the massive Holyoke basalt sheet.

The thickness, areal coverage, and direction of movement of five flows associated with the Mt. Tom plug have been mapped. Individual flows from this volcanic center are of two types: those containing 65 to 75 percent chlorophaeite, and those having felty texture with negligible original “glass.” Direction of movement of the flows was determined by plotting the preferred orientation of the (010) plane of feldspars. The complete geologic history of the Mt. Tom plug is presented in a series of sequential maps and sections.

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