Scharff, Skymour, and Newi'on — Castlepook Cave. 47 



set of passages running approximately 10° east of north are more numerous, 

 extensive, and regular, the cross-connecting series being fewer in number, of 

 more irregular form, and generally much smaller, sometimes mere tortuous 

 tunnel-shaped tubes through which one can pass only in a prone position. 

 The main set has generally a characteristic shape in cross-section, which may 

 be likened to a mushroom, consisting of two parts (see map ; fig. 3), an upper 

 helmet-shaped portion and a lower narrow and deep trench (the stem), a 

 nearly horizontal shelf occurring at the junction. In a few instances a 

 miniature replica is seen capping the principal cavity. 



From a study of the form of the passages, it becomes evident that they 

 were initiated by an underground drainage system which produced small 

 tunnels, the bottoms of which coincided with a main bedding plane of the 

 limestone (see especially fig. 4 on the map). A comparatively dry period 

 followed, during which the stalagmite floor was accumulated on top of a 

 sandy deposit containing numerous bones, belonging to various animal types. 

 A wet period supervened, which allowed of the enlargement of the old passages, 

 but on a lower level — down in fact to the next bedding plane, possibly 

 lower. In some cases the previously formed stalagmite floor has been thus 

 undermined, and has fallen down and become lodged in lower levels, and the 

 bones contained in it thereby become associated with bones of a more recent 

 period. There are some traces also of another stalagmite floor at a still lower 

 level, and apparently in situ. 



The highest stalagmite floor is, of course, the oldest, and those below 

 are (mostly reconstructed) more and more recent, so that there is an inverted 

 stratigraphical sequence as regards the fossils. 



The relative width of the upper and the lower portions of the typical 

 passage is capable of more than one explanation. 



Thus it might be the result of differential erosion on two beds of limestone 

 of very different solubility. The bed, however, in which the narrow lower 

 portion is mainly cut is occasionally excavated to the same width as the upper 

 portion. See fig. 2, for instance. 



I am inclined therefore to attribute the formation of the main tunnels 

 between the principal stalagmite floor and the conspicuous shelf some three 

 feet below, to the maintenance for a sufficient time of the outflow level or 

 discharge level of the water in circulation, at the level of the shelf. Under 

 such circumstances, any water below that level would be practically stagnant, 

 and would be quickly saturated, and thus incapable of further solution. 

 The upper and lower portions of the passages would, therefore, be forming at 

 the same period, but only in the upper part would considerable solution be 

 taking place. The lowering of the discharge-level would initiate solution at 



