Many interesting and perplexing questions arise in connection with the highly potassic volcanic associa- tion dominated by mafic and ultramafic rocks contain- ing leucite. Its occurrence is very restricted as compar- ed with the olivine-basalt trachyte kindred, but it is distributed at widely scattered points on all the conti- nents, and its chemical and petrographic individuality is both remarkable and constant. A considerable litera- ture is available related to the mineralogy, petrology, geochemistry, phase chemistry, distribution, and origin of this interesting suite of rocks. It seemed...