21. They resemble leucite in their shape, but have not yet been proved to have its crystalline outlines. 22. The former consist mainly of plagioclase, leucite and augite, while the latter contain olivine in addition. 23. By far the greater number of the rocks which contain leucite are lavas of Tertiary or recent geological age. 24. They are rich in leucite , but contain also some sanidine and often much nepheline with hauyne or nosean. 25. The leucite -tephrites and leucite-basanites of Vesuvius and Somma are familiar examples of this class of rocks. 26. The leucite-tephrites and leucite -basanites of Vesuvius and Somma are familiar examples of this class of rocks. 27. Rocks that contain leucite or nepheline, either partly or a wholly replacing felspar, are not included in this table. 28. The leucite tinguaites are usually pale grey or greenish in color and consist principally of nepheline, alkali feldspar and aegirine. 29. The leucite is often present in two sets of crystals, both porphyritic and as an ingredient of the ground mass. 30. Nepheline decreases in amount as leucite increases since the abundances of the two reflect the Na : K ratio of the rock.
