SPILITE, in petrology, a rock of basaltic character, but posses sing in place of the normal labradorite, a felspar of the composition of albite. The ferromagnesian mineral is an augite of pale brown colour; spilites are, however, usually very completely decomposed, augite being represented by chlorite and calcite. They are com monly vesicular or variolitic and show wonderfuly preserved pil low structure, a feature in most cases indicative of a submarine origin of the lavas. The individual pillows are filled with concen tric zones of vesicles filled with chlorite and calcite. The forma tion of pillow structure in volcanic rocks has been directly observed by Dr. Tempest Anderson in the case of the lava poured into the sea from the volcano Matavanu, in Samoa. On the surface of the lava, buds form and expand, giving rise to "pillows" with a glassy crust. The interspaces between the pillows are frequently filled with sediment representing the soft ooze of the sea floor. Not all pillow lavas are spilitic in character.
Some spilites showing pillow structure are not strictly lavas but shallow intrusions into unconsolidated submarine ooze. Excellent examples of such intrusive spilites are provided by the Devonian spilites of Nundle, New South Wales. Though the term spilite was first used by Brongniart for altered basic lavas free from phenocrysts and possessing well marked vesicular textures, a new significance was given to the term by Dewey and Flett, who urged that a special group of rocks—the spilitic suite—should be recog nized. According to these authors, a large suite of igneous rocks is
genetically associated with the spilites. Their composition varies widely, but they all have some characters in common. They possess a high percentage of soda and are usually extensively al tered. These rocks are albite-dolerites, minverites, picrites, kera tophyres, soda-felsites and soda-granites. The spilitic suite was originally given a status comparable with that of the alkaline and calc-alkaline suites. The spilitic series is regarded as peculiar to districts that have undergone a long continued and gentle subsid ence with few or slight upward movements and no important faulting. The lavas are poured out on the margins of geosynclines while their common sedimentary associates are black shales, lime stones and radiolarian cherts. As geosynclines are the centres of subsequent fold movements many ancient spilites have become extensively folded and metamorphosed with production of new minerals, including albite of metamorphic origin. Eruptions hav ing the spilitic facies have occurred repeatedly over a wide area and on a large scale. The analyses of average basalt and spilite are as follows: