IT may be of interest to geologists to know that I have lately ascertained that the beds of chert which occur in the limestones of the Yoredale series of Yorkshire are distinctly of organic origin, and that, in fact, they are composed of the heterogeneously-mingled spicules of disintegrated siliceous sponges. The beds vary from 3 inches to 18 feet in thickness, and the limestones in which they are interbedded are nearly exclusively composed of the broken-up remains of crinoids, thus showing a well-marked alternation of periods in which sponges and crinoids succeeded each other. The spicules can only be studied in thin microscopic sections of the rock; in some cases they are very perfectly preserved and their axial canals are clearly shown; in other examples only very faint outlines can be made out. They appear to belong for the most part to the same group of Hexactinellid sponges as the recent genus Hyalonema but Monactinellid spicules, like those of the existing genus Reniera are also very numerous in some of the beds. Such an enormous accumulation of the debris of siliceous sponges proves that these organisms were as abundant in the Carboniferous as in the Cretaceous seas.