For a number of reasons special interest attaches to the geological features of the Leucite hills of Wyoming. The hills present a series of rock-types which, on the petrographical side, are unique and extremely important in the systematic study of the eruptive rocks. The rocks were the first with leucite discovered in America and, except for one occurrence in the Dutch Indies, the first to be observed outside the continent of Europe. The obvious joy of Professer Zirkel in recognizing them, as shown in his classic report on the rocks of the fortieth parallel, still arouses a responsive chord in the minds of the later, and above all the younger, workers who read his pages. They are the only eruptive rocks known to have original phlogopite, as Doctor Cross has remarked, and a rare hornblende in some of their flows still invites further investigation.
The petrographical characters, however, . . .