Thrown out promiscuously from active vents, the materials that form tuffs arrange themselves, on the whole, according to relative size over the surface on which they come to rest, the largest being generally grouped nearest to the focus of discharge, and the finest extending farthest from it. As the volcanoes of which records have been preserved among the geological formations were chiefly subaqueous, the fragmentary substances, as they fell into water, would naturally be to some extent spread out by the action of currents or waves. They would thus tend to take a more or less distinctly stratified arrangement. Moreover, as during an eruption there might be successive paroxysms of violence in the discharges, coarser and finer detritus would successively fall over the same spot. In this way, rapid alternations of texture would arise (Fig. 13). A little experience will enable the observer to distinguish between such truly volcanic variations and those of ordinary sedimentation, where, for instance, layers of gravel and sand repeatedly alternate. Besides the volcanic nature of the fragments and their non-water-worn forms, he will notice that here and there the larger blocks may be placed on end--a position the reverse of that usual in the disposal of aqueous sediments, but one which is not infrequently assumed by ejected stones, even when they fall through some little depth of water. Further, the occurrence of large pieces of lava, scattered at random through deposits of fine tuff, would lead him to recognize the tumultuous discharges of a volcanic focus, rather than the sorting and sifting action of moving water. Admirable illustrations of these various characteristics may be gathered in endless number from the Palæozoic volcanic chronicles of Britain. I may especially cite the basin of the Firth of Forth as a classical region for the study of Carboniferous examples. [Illustration: Fig. 14.--Alternations of Tuff (_t_, _t_,) with non-volcanic sediment (_l_, _l_).]