Total alkali (Na<sub>2</sub>O + K<sub>2</sub>O) versus silica (SiO<sub>2</sub>), or TAS, classification system of igneous rocks. After Le Maitre (2002).
W. Ian Ridley
2012
<a rel="nofollow" class="external text" href="https://pubs.usgs.gov/sir/2010/5070/c/">W. C. Pat Shanks III and Ronald Thurston (editors), <i>Volcanogenic Massive Sulfide Occurrence Model</i></a>, US Dept. of the Interior, U.S. Geological Survey Scientific Investigations Report 2010-5070-C (2012). p. 237 (fig. 15–2 A)