Nevertheless, this absence of arrangement sometimes gives place to a stratification which becomes more distinct in proportion as the material of the vent passes from coarse agglomerate into fine tuff. It is possible that the existence and development of this structure depend on the depth at which the materials accumulate in the funnel. We may conceive, for instance, that in the lower parts of the chimney, the stones and dust, tumultuously falling and rebounding from projections of the rugged walls, will hardly be likely to show much trace of arrangement, though even there, if the explosions continue to keep an open though diminishing passage in the vent, alternations of coarser and finer layers, marking varying phases of eruptivity, may be formed in the gradually heightening pile of agglomerate. Rude indications of some such alternations may sometimes be detected in what are otherwise quite unstratified necks. In the upper part of a volcanic funnel, however, close to and even within the crater, the conditions are not so unfavourable to the production of a stratified arrangement. As the pipe is filled up, and the activity of eruption lessens, explosions may occur only from the very middle of the orifice. The debris that falls back into the vent will gather most thickly round the walls, whence it will slide down to the central, still eruptive hole. It will thus assume a stratified arrangement, the successive layers lying at the steepest angles of repose, or from 30° to 35°, and dipping down in an inverted conical disposition towards the centre. If the process should continue long enough, the crater itself may be partially or completely filled up with detritus (Fig. 25). Of this gradual infilling of a volcanic chimney with stratified agglomerate and tuff, examples belonging to different geological periods will be cited in subsequent chapters. I may here especially allude to one of the most recently observed and best marked illustrations, which occurs on the west side of Stromö, in the Faroe Islands (see Figs. 310, 311, 312). A neck has there been filled up with coarse agglomerate, which is rudely stratified, the layers dipping steeply into the centre, where the tumultuous assemblage of large blocks no doubt points to the final choking up of the diminished orifice of explosion. The walls of the neck are nearly vertical, and consist of the bedded basaltic lavas through which the vent has been opened. They terminate upward in a conical expansion, evidently the old crater, which has subsequently been filled up by the inroads of several lava-streams from adjacent vents. It is here manifest that the bedded agglomerate belongs to the uppermost part of the volcanic funnel. [Illustration: Fig. 25.--Neck filled with stratified tuff. A. ground plan; B. transverse section.]