Geological significance of lead-alpha and isotopic age determinations of "alkalic" rocks of New England

Recent age determinations indicate that at least two groups of "alkalic" igneous rocks exist in New England, with ages of about 185 and 270 million years. Because of their lithologic and geologic similarities, all these rocks had previously been grouped with the White Mountains plutonic-volcanic series of New Hampshire. Until reliable petrographic or geologic criteria become available for distinguishing these age groups, caution should be exercised in assigning any undated body of "alkalic" rock to one or the other group.