PHYSICAL HISTORY AND GEOLOGY. 29 



mon sight along the roadsides. The process of the binding 

 together of the loose particles of debris which are to constitute 

 a rock is sometimes a very rapid one, especially along the 

 water's edge, and may be followed in its different stages. 



The basal rock of the cliffs, especially on the south shore, is 

 in places excessively indurated, and about as resisting as a 

 non-siliceous limestone can well be; when struck with a ham- 

 mer it at times rings with all the intensity of the volcanic ring- 

 ing rocks, and chips off as sharp-edged flakes. The granular 

 structure which is so prominent in the softer rock may be re- 

 tained, but it sometimes largely or wholly disappears, and 

 the mass appears to be homogeneously compact. The matter 

 of hardness or compactness is, however, not one necessarily 

 depending upon age, since we often find the tougher rock oc- 

 cupying the high level, and-overlying the softer rock below. 

 At other places the two kinds of rock alternate with one 

 another. From the constancy of its occurrence at, or near, 

 the base of the island,' the hard subcrystalline limestone is 

 locally known as the " base rock ; " it serves largely, but by no 

 means invariably, to distinguish the old beach formation, and 

 thus to locate the former sea-border. The same rock forms 

 the lower moiety of the three pinnacles of the North Rock. 



The constituent particles of the softer limestone are of about 

 the size of a pin's head, or smaller, among which the debris 

 of shells and millepores are distinctly recognizable. Coral 

 fragments are apparently much less abundant, and, indeed, 

 it was only with diflSculty that I determined these at all, 

 except where, at long intervals, fragments of large size were 

 caught up in the mass. Possibly, the finer undefined particles 

 may have been those of corals, whose cellular structure would 

 have readily lent itself to a powdering such as would com- 

 pletely efface determining characters. Still, the fact remains 

 that much, or most, of this rock is made of millepore and shell 

 fragments, and whatever part corals may have taken in its 

 formation, it cannot be considered to be a simple coral rock. 

 The examination of the long stretch of beach which faces the 



