MINERALOGY OF THE REDHEAD. 



351 



posed for some time to the action of the atmo- 

 sphere, they acquire a considerable degree of hard- 

 ness. When a fresh piece of the rock is put into 

 a glass with water and agitated, the cohesion be- 

 tween the particles is destroyed, and when the 

 fluid is suffered to rest, the ingredients of the stone 

 divide themselves into three portions. At the 

 bottom is found a fine gritty sand, in the middle 

 small scales of mica, and at the top a portion of 

 argillaceous matter, which continues longest sus- 

 pended in the water. The hardness which the 

 stone acquires by exposure to the atmosphere, is 

 probably owing to the desiccation of this last sub- 

 stance. In these soft strata, water-borne balls of 

 quartz, granite, gneiss, mica-slate, porphyry, and 

 trap, lie scattered without order, and in some 

 places they are accumulated in such quantity, as 

 to constitute the rock to which Colonel Imrie has 

 attached the very appropriate name of graveistone. 

 Sometimes these strata include beds of coarse 

 sandstone, composed of angular grains of quartz 

 and felspar with a little mica, which are less liable 

 to detrition from the action of the sea. 



Immediately to the north of the harbour, simi- 

 lar strata prevail, but the quantity of gravel im- 

 bedded in the strata is much greater, and the beds 

 of gravelstone more numerous. In some in- 

 stances, these rounded balls, which are from a 

 line to a foot in diameter, are cemented to- 



