Grey Rock and Thyme 179 



grandest dales and moors. The cool, grey rock, 

 riven so boldly, and as though of its own pleasure, 

 into its precipices and gleaming scarps, carries in 

 the midst of its rugged and solid grandeur a sense 

 of the strange corroding forces which make it porous 

 to its very heart. The famous cloven valleys of 

 Derbyshire and the Mendips, and the many scars 

 and crags of lonelier and more isolated moorlands, 

 are only the external gravings of a vast and secret 

 process of natural disintegration which hollows the 

 core of the rock. The clear and thyme-strewn 

 sheep walks are raised to their open place in heaven 

 on an underworld of caverns and pillared fissures. 

 Comparatively few of these have yet been explored 

 by man, though in many of the world's great lime- 

 stone regions the caves that have been tracked 

 and rendered accessible rank among the greatest 

 of natural marvels. Bearing the acid from the 

 atmosphere that eats out the lime of the rock, the 

 subterranean rivers are still endlessly developing 

 their profound recesses. Many of the known ranges 

 of caverns in the Mendips and the Pennines have 

 never yet been fully traced, and there may be others 

 even vaster to which no access is known. In the 

 caves which have from time to time been threaded, 

 after leading the explorer on from hall to hall, 

 and shaft to shaft, the stream may finally defy 

 him by plunging down a vertical rift into unknown 

 depths. 



Even in the outer sun, on the sheep-cropped turf 

 of the hills, the swallow-holes, where the streams 

 descend by their Avernus, have a strange and 



