Formation of "Natural Quarries." 163 



Burface by capillary attraction and by the evaporation of such 

 water with deposition of mineral water, this detritus may be 

 compacted and hardened by the introduction of (chiefly) iron oxide 

 or travertine as a cement. The cemented detritus then forms a 

 ■cover, fairly strongly resistant to erosion, over the underlying soft 

 rocks. The latter are, therefore, so long as the cover lasts, pro- 

 tected from further erosion. The cover may, however, not be quite 

 continuous everywhere, or it may be very thin and not completely 

 cemented in certain places, or for some other reason a hole in such 

 cover may exist or be made. Having such a hole, the rain may 

 directly beat upon the soft underlying rocks, or may find its way 

 between the cover and the softer rocks, with the result in either 

 case of removing portions of such softer rocks and undermining 

 the cover, which collapses and gradually disintegrates into fine 

 enough material to be carried away by the rain. Once started this 

 process may go on, the hole growing larger until one sufficiently 

 large enough is produced to be called a circular natural quarry. 

 This seems to be the chief method of formation, although some 

 quarries occur which do not seem to have had a distinct cover; but 

 here there is probably some surface hardening of the rocks, with- 

 out, however, the formation of an appreciable cover. It is also 

 conceivable that some parts of the soft rock are less resistant to 

 •erosion than others; that therefore where no cover exists, the beat- 

 ing action of the rain may gouge out the less resistant rocks; and 

 that when once started the cavity so formed may grow in size. 



The rectangular quarry in places resembles the scar left by a 

 landslip, but as an accumulation of detritus, such as would result 

 from a rock fall, is never found on the floor of the quarry, this 

 mode of origin must be rejected. It is difficult to account for all 

 quarries of this type, but the following conditions favour their 

 formation : — (1) An abuttal along a vertical or nearly vertical 

 plane of decomposed soft rocks against a band of hard erosion- 

 resisting rocks. (2) Decomposed soft rocks capped by a practically 

 continuous band of loose fragments of a hard erosion-resisting 

 rock derived from an outcrop farther up the hillside. In both 

 cases, if the soft rocks form vertical or nearly vertical schists, the 

 formation of the type of quarry now discussed, is accelerated. 

 The mode of origin is not easy to understand, but it seems that the 

 action of rain l>eating on the soft i-ocks is mainly responsible for 

 the wearing away, aided of course by the ordinary weathering 

 agents ; and that the resulting form is governed by the band of 

 hard rock or by the surface cover of hard detrital rock mentioned 



