FELSITE. As a rock name, felsite is rather indefinite, but is a very convenient name for those dense fine-grained igneous rocks, usually gray but often red or green, that to the naked eye show no crystals of minerals. Fel sites generally are ancient lavas containing a high per cent of silica, and are composed almost wholly of minute crystals of quartz and feld spar. The percentage of silica varies, and the relationship and probable origin of a felsite can be determined by analysis and examination with the microscope. Igneous rocks having a finely crystalline ground mass with visible crystals are said to have a felsitic ground mass, thus petrographers speak of felsite-porphyry. See ROCK ; RHYOLITE ; TRACHYTE.
FELT,Joseph Barlow, American historian: b. Salem, Mass., 22 Dec. 1789; d. there, 8 Sept. 1869. He was graduated at Dartmouth College 1813 and was ordained Congregationalist minis ter, having churches in Sharon, Mass., and
Hamilton, Mass. In 1836 he was appointed to classify the Colonial and State archives of Massachusetts. His 10 years' work at this task was invaluable to the State, and he was recog nized as an authority on New England history. He was recording secretary of the American Statistical Association 1839-59; librarian of the Massachusetts Historical Society 1842-58; and president of the New England Historic-Genea logical Society 1850-53. Among his works are 'Annals of Salem' (1827) ; 'History of Ips wich, Essex and Hamilton' (1833); 'Histori cal Account of Massachusetts Currency' (1839) ; 'Ecclesiastical History of New England' (1855 62) ; and 'Collections for the American Statis tical Association do Towns, Population, and Taxation) (1847).