CHARNOCKITE, in petrology a series of igneous rocks (originally described by Sir T. H. Holland) from Madras presi dency, southern India, and forming a well-defined petrographic province of Archaean age. The name is derived from that of the founder of Calcutta, job Charnock, whose tombstone is made of a typical member of the series. The series includes a wide range of rock types from ultrabasic pyroxenites through intermediate types—norites and quartz-hypersthene-diorites—to acid pyroxene granites. The term charnockite is often specifically reserved for the acid hypersthene-granite. One of the distinguishing features of the series is the recurrence in many members of a strongly pleochroic red-to-green hypersthene. The typical members of the series, however, show other peculiar mineralogical features, namely the development of schiller structures, from the presence of minute plate or rod shaped enclosures disposed parallel to definite crystallographic-planes or axes. The optical effect of these enclo sures is seen in the blue opalescence of the quartz, the milky shim mer of the felspar and the bronzelike lustre of the rhombic pyroxene. Myrmekitic, microperthitic and antiperthitic structures are common in the felspars. In the soda-lime felspars there is a striking tendency to absence of the usual twinning lamellae. The members of the charnockite series frequently show a banded or gneissic structure, now usually interpreted as a flow banding, due to movement during the epoch of crystallization. Chemically the series is distinctly subalkaline, with a dominance of iron oxides over magnesia and lime.
The various members are of widespread distribution and great petrological importance. In southern India they occur in the Nilgiri hills, the Shevaroys and the western Ghats, extending southward to Cape Comorin and reappearing in Ceylon. They occur in the Archaean shield of Western and South Australia and in Adelie Land, Antarctica. Similar rocks are known from the Ivory coast of West Africa, the eastern part of Ellesmere Land; form the Cortlandt series near Peekskill, N. Y., and recur at other localities in the eastern United States and Canada. The most note worthy occurrences in the northern hemisphere are found in south western and western Norway, at East Kersund and Soggendal, and over a large area in the Bergen and Jotunheimen districts. The majority of the known occurrences of the charnockite series are of Archaean age, but it is probable that the anorthosite-char nockite series of the Bergen-Jotunheimen districts is of lower Palaeozoic (Caledonian) age. (C. E. T.)