RAMBLES OF A GEOLOGIST. 439 



ing around it, gives the waves scope to gather bulk and mo- 

 mentum, may be found the secret of the extraordinary height 

 to which the surf sometimes rises against its walls of rock. 

 During the fiercer tempests, masses of foam shoot upwards 

 against the precipices, like inverted cataracts, fully two hun- 

 dred feet over the ordinary tide-level, and, washing away the 

 looser soil from their summits, leaves in its place patches of 

 slaty gravel, resembling that of a common sea-beach. Rocks 

 less perpendicular, however great the violence of the wind 

 and sea, would fail to project upwards bodies of surf to a 

 height so extraordinary. But the low angle at which the 

 strata lie, and the rectangularity maintained in relation to 

 their line of bed by the fissures which traverse them, give to 

 the Orkney precipices, remarkable for their perpendicular- 

 ity and their mural aspect, exactly the angle against which 

 the waves, as broken masses of foam, beat up to their great- 

 est possible altitude. On a tract of iron-bound coast that 

 skirts the entrance of the Cromarty Frith I have seen the 

 surf rise, during violent gales from the north-west especially, 

 against one rectangular rock, known as the White Rock, fully 

 an hundred feet; while against scarcely any of the other 

 precipices, more sloping, though equally exposed^ did it rise 

 more than half that height, 



