THE PETROSILEX PROBLEM

Petrosilex is a somewhat archaic term used by Massachusetts geologists in the 19th century to describe a siliceous felsite type of rock. First use was by Hitchcock, then by Crosby, and picked up by Bouve in his history of Hingham. This terminology was also used by British geologists to describe the entire domain of siliceous rocks. Both Billings and Kaye emphasized silicious rocks in their caldera collapse theory of the Mattapan-Lynn volcanics. While petrosilex is described as a most beautiful rock, the real interest, however, comes from its recognition as a unique stage in the evolution of continental margins. Curious petrologies such as alaskite may also be considered in the petrosilex problem. Examples from field and core studies by the author for the Greenbush railroad line, Northboro bridges, Metrowest tunnel, combined with wanderings in the Middlesex Fells and gravestone observations at Saugus and Marblehead are provided in the discussion. This paper reviews the historical geology of thought on pre-Cambrian siliceous rock zones from geosynclinal to plate tectonic to large igneous province and continental breakup theory today.