Manganiferous rocks are found in two areas in eastern Aroostook County, Maine, one west and northwest of Presque Isle and the other south of Houlton. The manganiferous deposits are sedimentary lenses, up to 150 feet thick and half a mile long, in middle Silurian argilllte. The rocks of both areas are much deformed, but the deformation is less intense and the continuity of the deposits therefore greater in the Presque Isle area than in the Houlton area. The dip of the rocks at most deposits is steep to vertical.

Below the zone of oxidation the manganese occurs as carbonate and as the silicates braunite and bementite. These minerals are primary constituents of the shales and should continue downward to the limits of the sedimentary units that contain them. One deposit, which has been explored by drill- ing to an average depth of 130 feet, contains about 11 percent of manganese, but most of the deposits probably contain be- tween 6.5 and 9 percent. There are many millions of tons of these manganiferOus shales in the county. Some of the deposits contain 20 percent or more of iron., mostly as hematite and limonite, and magnetite is abundant in the Houlton area.

In parts of the deposits above the water table about 30 percent of the manganese occurs as a heavy stain of secondary oxides along joints and parting planes. . The manganese oxides partly replace the primary rock minerals for a distance of 1 to 3 millimeters from cracks in the rock. At the one deposit from which drilling data are available, manganese oxides are found on only a few fractures below the water table, which is at a depth of 20 feet, and none were seen in rocks from below 45 feet. It is estimated that there may be over 1,700,000 tons of partly oxidized manganiferous shale in the region.

Some of the deposits are overlain by a thin zone, probably about a foot thick, that is richer in manganese than the underlying oxidized rocks. At the deposit explored by drill- ing, the average manganese content of the deposit as a whole is 76.4 percent of the average as determined from surface sam- ples. The surface enrichment is believed to have taken place before the last advance of the Pleistocene ice, which may have scraped off the enriched part of many deposits. There are probably less than 100,000 tons of enriched rock in all the scattered deposits of the region.

There is a very much greater tonnage of low-grade unenriched manganlferous rock in Aroostook County, but it cannot be utilized until methods of extraction have been improved.