234 On the Alpine Glacier, Iceberg, QNo. 159. 



They not only traverse the upper horizontal surface of the strata, 

 but sometimes continue over the edges down their vertical extremity 

 or sides, which is attributable to the action of water slowly trickling 

 over the edge, and not propelled beyond the edge to a distance from 

 the vertical side, as is the case in a cascade. 



The water, in many instances, seems to have acted corrosively as 

 well as erosively on the substance of the limestone ; for in examin- 

 ing some rain water, which had lodged in one of the eroded cavities, 

 I found it held a considerable quantity of lime in solution. Carbonic 

 acid might have been supplied from atmospheric exposure or from the 

 surrounding dense vegetation, which the rains refresh. The solvent 

 power of water too in tropical climates is considerably enhanced, not 

 only by the increased temperature of the water itself, but by expan- 

 sive action of the sun's rays on the atoms composing the rock-bare 

 surfaces, some of which I have found often heated to 130°. The solid 

 layers of schist are free from such furrows, but have a scabrous water- 

 worn appearance, as if the limestone had been washed away. 



Any pre-existing cavity in the surface of the rock forming a lodge- 

 ment for the water, assists in the erosion of hollows. Strings of iron 

 pyrites frequently drop out in weathering, leaving a chain of oval 

 cavities, which the water soon works down into a continuous furrow. 

 Others commence in the perforations of lithodomous molluscs, or those 

 of existing snails which apparently by the chemical action of their 

 juices take up the lime necessary for their house and food, and are 

 found in numbers adhering to the surfaces and sides of the lime- 

 stone. 



It is evident, however, that some of the furrows were scooped out 

 prior to the last displacements of the rock strata, as they partake of 

 the faults and dislocations ; and it is probable they were formed during 

 the elevation of the land by sea water, as it is well known that sea 

 water by the decomposition of its muriates and sulphates produces 

 furrows and wrinkles on the surface of limestone, particularly near 

 the water's edges, and subsequent rains have no doubt acted in extend- 

 ing and modifying them. The entire absence or great comparative 

 rarity of such furrows on the surface of the associated sandstone, may 

 be regarded as a further indication of the chemical action of the water 

 in producing the furrows on the limestone. 



