DILUVIAL DETRITUS IN 
70 
whilst in others that traverse the rock obliquely it can he seen only 
where their upper extremity intersects the surface of the rock, and if 
this happens not in a cliff, but along the level face of the country, it 
is usually so completely filled and covered up with earth as not to be 
discoverable unless by approaching it through the caverns from below. 
We have in this circumstance a satisfactory reason why so many 
cavities, having at first view no apparent communication with the 
upper surface, are intersected in working the central and deepest 
portions of these limestone rocks * 
In almost all the cavities there occurs a deposit of diluvial detritus, 
consisting of mud and sand, and angular fragments of limestone ; 
these substances sometimes entirely fill up the lower chambers, and are 
usually lodged in various quantities and proportions on the shelves and 
ledges, and lateral hollows of the middle and upper regions. The com¬ 
position of the mud, or earthy portion of this diluvium at Plymouth, 
differs in some degree from that of the cave at Kirkdale, having 
been derived from the detritus of strata of a different character ; it is 
of a redder colour, and looser texture, and less calculated to protect 
the bones in it from the access of atmospheric air and water. In one 
large vault at Oreston, where the quantity of diluvium was very great, 
it was stratified, or rather sorted and divided into lamina; of sand, earth, 
and clay, varying in fineness, but all referable to the diluvial wash¬ 
ings of the adjacent country. It is often partially interspersed with 
small fragments of clay-slate and quartz. The sand and loam are in 
many places invested with, and cemented together by, stalagmite, but 
* See a good illustration of this in Plate XX., and the description of the cave at the 
Dream Mine, near Wirksworth, at page 62, et seq. 
