84 PREHISTORIC EUROPE. 



period of deposition. Now and again, however, we find them 

 showing numerous intercalations of earth — some of which cer- 

 tainly point to the former presence of muddy water. An 

 excellent example of this was met with during the exploration 

 of Brixham Cave, in one part of which six or seven plates of 

 crystalline, compact, soil-stained, finely -laminated stalagmite, 

 varying from half an inch to upwards of an inch and a half in 

 thickness, extended horizontally from wall to wall, one over the 

 other, and alternated with an equal number of interstratified 

 layers of earth of similar thickness. Again, stalagmites, so far 

 from being always comparatively pure, are often so highly im- 

 pregnated with earthy ingredients as to assume the character of 

 calcified earths. Such impurities may have been introduced in 

 various ways. Most limestones when they are dissolved in car- 

 bonic acid leave a red residue behind, and there can be little 

 doubt that much of the earthy matter in stalagmitic accretions is 

 of this nature, and to that we may add the red earth, mud, and 

 silt introduced by rains and freshets through fissures in the roofs 

 and sides of caves, and even in many cases by their more open 

 mouths. Some of the caves in the Eock of Gibraltar bear 

 evident marks of having been invaded in this manner. The 

 heavy rains that fall on the western slopes of that ridge rush 

 down the rocky declivities, sweeping before them considerable 

 quantities of red earth, derived from the subaerial decomposition 

 of the limestone, and much of this muddy water escapes into 

 underground cavities through narrow fissures, and now and then 

 pours into the caverns by their chief entrances. Finally, when 

 we conceive of the caves as having frequently been the actual 

 abodes during long periods of various wild beasts and men, we 

 can have no difficulty in understanding how stalagmitic accre- 

 tions might come to be soil-stained, even although rain and 

 freshets never found access to them at all. 



I have mentioned the fact that stalagmites often pass into 

 what might be termed calcified earths, and from what has been 

 said about the origin of such impurities the reader will be pre- 

 pared to learn that frequently the floors of our old limestone- 



