146 DESTROYING INFLUENCES OF FROST AND RAIN. 



millstone-grit, are decomposed in lines corresponding to the differences 

 in the composition of the stone. 



In these cases the stone appears to undergo gradual and continual 

 waste, but sometimes the whole surface exfoliates. Basalt very fre- 

 quently suffers this kind of waste, granite not rarely ; and it has been 

 sometimes supposed in these instances, that the atmospheric action 

 merely discloses the incipient concretionary structure of the rocks. 



From Frost. Frost is likewise an important agent in reducing to 

 smaller masses the materials of the earth. Some stone, if brought to 

 the surface in winter full of its " quarry water," will break in pieces 

 directly. Advantage is taken of this circumstance by the slate-workers 

 of Stonesfield and Collyweston, who quarry their stone in the winter, 

 taking care to shield it from the sun and the wind till the frost has 

 acted upon it, with the aid of water, if necessary, which, by disclosing 

 the natural fissility of the stone, permits the blocks to be cleft into 

 thin, sound roofing - slate. Landslips in mountainous regions are, 

 probably, much accelerated by the power of frosts. In ascending the 

 Righi from Waggis, on the Lake of Lucerne, we are much struck by 

 the extraordinary length and continuity of the joints of the nagelflue. 

 It is from these natural partings that the landslips fall, when repeated 

 rains, snows, and frosts have worn or burst them open, and the water 

 passing down them undermines the foundation of the cliff. Thus 

 huge blocks, liberated from their attachments, roll down the steep 

 descent, or half the summit of a mountain slides upon its argillaceous 

 bed Vast portions have thus slipped from the Righi towards the 

 isthmus which divides the lakes of Zug and Lucerne, and others are 

 preparing to follow. 



Effects of Rain. We come now to the effects of rain, and without 

 dwelling on the general degradation of the softer surfaces of the earth 

 caused by this agent, we shall proceed to show, that within the his- 

 toric era hard and dura'ble stones have been greatly furrowed by the 

 rain ; and that in more ancient periods, the precipitations from the air 

 have carved channels of various kinds, and sometimes formed real 

 though miniature valleys of great length and continuity. 



On Monumental Stones, &c. Many Druidical monuments in the 

 north of England are constructed of coarse millstone - grit, a rock 

 admirably suited for yielding those enormous blocks preferred by the 

 ancient architects. Three huge Druidical stones, now standing near 

 Boroughbridge, called the " Devil's Arrows," present us with an in- 

 structive lesson on the ultimate fate of all human erections exposed 

 to the ravages of time. 



The rain, beating for 2000 years upon these venerable pillars, 

 has cleft their tops, and ploughed deep furrows down their sides. 

 The grooves are deepest at the top, and become wider and less distinct 

 towards the bottom ; they cross indifferently the false-bedded layers 

 of pebbles, and go directly downwards. One of the stones leans re- 

 markably and threatens to fall, but an examination of the furrows 

 shows the inclination to be of most ancient date, for they descend much 

 farther down the pillar on the upper inclined face than on the under. 



