JONES — ON THE WEATHERING OF GRANITE, 



311 



sometimes scarcely coliereiit, and, on exposure to tlie weather, soon 

 falling away. Along the belt w^here the basins exist, the gTanite is 

 for the most part more liable to decomposition than at the harder 

 and more crystalline tors. This is sho^vn by the many rounded tors, 

 and every roadside-cutting shows the rapidity of the decay. The 

 division of the gi^anite into tabular sheets of rock of iiTegTdar thick- 

 ness, causing the appearance of stratification, is common to all the 

 gTanite of this district. In irregularities on the surface of the 

 granite, and in hollows, very probably in many cases caused by the 

 nodules above noticed, water lodges on and penetrates into the 

 porous granite, and the decay thus commenced will gradually enlarge 

 the cavity to a basin. During the inclement part of the year these 

 basins are fall of water, that during part of the time often rapidly 

 alternates with ice. When the wann weather comes on, the water 

 evaporates, and the basins are dried up ; from the frequent showers 

 there is, then, a constant change between the rock being saturated 

 with wet and it being warm and dry. The gradual action of the 

 water is very perceptible ; when it has evaporated, the stone up to 

 the water-line is left a lighter shade than the adjoining rock ; the 

 felspar-crystals instead of presenting their usual appearance, are dull 

 and full of minute cracks, and appear as if about to fall into small 

 fragments similar to those found in the basins ; the action of the 

 water is evident to the eye though not easily described. An un- 

 broken face of granite resists the weather more powerfully than the 

 rock does when it is broken or penetrated ; in those cases the water 

 soaks into the granite, and thus renders it more easily acted upon by 

 the alternations of heat and cold, wet and dryness. Such action 

 when once commenced wiU continue until checked by the unbroken 

 face of a parting which will limit the extension either perpendicularly 

 or horizontally. The tabular formation of the granite is probably 

 the cause of the frequent occmrence of the basins with flat bottoms. 

 The gradual decay thus acting from a centre will cause the nearly 

 circular and oval forms that so many of the basins present, the 

 variation from that shape being probably caused by a difference in 

 the structure of the granite. The eye will in a short period dis- 

 criminate between the tors where basins would probably be found or 

 not. Firstly, where it is the character of the tor to have the per- 



