“…The Northfield slate (Currier and Jahns, 1941, p. 1501-1506) is a quartz-sericite slate (table 2, column 14) with widely spaced interbeds of siltstone and silty crystalline limestone that are a few inches thick. Minor constituents of the slate include carbon, ilmenite, porphyroblastic ankerite, and tourmaline.…”