A western naturalist once said that the geology of Kansas was monotonous. In one sense this remark is certainly justifiable, and the same may be said about the geology of some of the other States on the Western plains. The American continent is built on a comprehensive plan, and many of its formations can be followed for hundreds of miles without presenting much variation in general appearance. Occasionally, however, some feature of special interest crops out from the serene uniformity, and the very nature of its surroundings then makes it appear all the more striking. Minor accidents in the development of our extensive terranes sometimes stand out in bold relief, as it were, from the monotonous background. In their isolation from other details such features occasionally display past events with unusual clearness. Such is the case with a deposit of volcanic ash which has been discovered in the superficial strata on the plains.[31] It lies scattered in great quantities in a number of localities in Nebraska, Kansas, South Dakota, and Colorado, having been found in no less than twenty counties in the first-mentioned State. It measures from two to fourteen feet in thickness in different localities, and is mostly found imbedded in yellow marl and clay, and has a somewhat striking appearance in the field, due to its snowy whiteness and to the sharpness of the plane which separates it from the underlying darker materials. Many years before its real nature was known it had been noticed and described by Western geologists. Prof. O. T. St. John saw it many years ago in Kansas, where it appeared as "an exceedingly fine, pure white siliceous material," forming a separate layer of several feet, and set off by a sharp line from the buff clay-marl below. His words describe its usual appearance in other places (see Fig. 1). [Illustration: FIG. 1.--STRATIFIED VOLCANIC ASH NEAR MEADE, KANSAS. (From the University Geological Survey of Kansas, vol. ii.)] This ash occurs in several outcrops in McPherson County in the central part of Kansas, where the writer had an opportunity to study it somewhat in detail a few years ago. Some of the features of the dust at this place reveal the conditions under which it was formed with considerable distinctness, and the volcanic episode which produced it appears strikingly different from the dull monotony of the ordinary geological work recorded in the terranes of the plains. It may be said to consist of angular flakes of pumice, averaging one sixteenth of a millimetre in diameter, and having a thickness of about one three-hundredth of a millimetre. The most common shape of the flakes is that of a triangle, or rather of a spherical triangle, since the flakes are apt to be concave on one side and convex on the other. In the microscope they sometimes appear like splinters of tiny bubbles of glass, and this is really what they are (Fig. 2).