Watchung Mountains - Mineralogy

Mineralogy

The Watchungs are composed principally of volcanic basalt, which historically has been used in railroad beds and road construction. In addition to this, in many places the mountains are underlain by red and white sandstone which has at times been used in building construction. Mica and calcareous spar often accompany these sandstone beds.

Due to the volcanic nature of the Watchungs, zeolites, including prehnite, analcime, and stilbite, which form from a reaction of mafic rocks in alkaline environments, can be found along exposed ridge lines. Agate, primarily in the form of chalcedony, and crystalline quartz (sometimes in the form of amethyst), are prominent in the ancient lava flows of the Watchungs and are typically seen as embedded nodules along exposed fronts. Datolite, another nodular mineral, has been found embedded in the volcanic rock around the Great Falls. Additionally, jasper and satin spar are known to exist within the northwestern Preakness Range.

Copper also can be found in the Watchungs. Near Belleville, ore containing 8% copper was discovered, and a copper mine once operated in the area. Other copper bearing ores have been noted near Paterson. These ores typically contain cuprite (red copper oxide) and/or copper carbonate in a matrix of red or gray sandstone. Pyritous copper, also known as chalcopyrite, is not known to exist in ores found in the Watchungs.