70 
ST. HELENA. 
nation to be very different from those already passed, and, being com- 
posed of phonolite or clinkstone with a very whitish-grey fracture 
and a perfectly white marginal surface, afford a good illustration 
of the decomposition which atmospheric influences are working on 
their exterior. These masses of rock have rolled down several 
hundred feet from the dike which crosses the crater-edge above, 
while, on the other side also, similar boulders have fallen down into 
the crater, where, on the steep sides, they lodge in strange positions. 
Along the road to Lot’s Wife Wood, just after passing Lufkins, 
one or two of these great stones, measuring some forty or fifty feet 
in length, may be seen perched up in most curious positions. 
Further still to the westward, on the main ridge or crater edge, 
occurs the third dike to which allusion has been made. It is com- 
posed of altogether a different material from the others, being of a 
dark blue basalt, with embedded crystals of augite and olivine ; 
boulders have also become detached from it, and rolled down the 
sides of the hill ; many of them, weather-worn into fantastic shapes, 
have lodged on a plateau, and, through their resemblance to 
mammoth tombstones, have obtained for the locality the name of 
“ The Churchyard.” 
It is the great Lot dike, however, which is best suited for 
illustrating our present subject. The average height of those 
elevated columnar portions of it, called Lot and Lot’s Wife, above the 
adjacent ground which has been worn away, is 275 feet. They are, 
as has been mentioned, formed of a hard felspathic finely crystalline 
greystone, and would, as is seen to be. the case, naturally wear down 
more slowly than the surrounding rocks, which are much softer and 
altogether of a different composition. Supposing, then, the latter to 
weather away at the rate of one-tenth of an inch faster each year 
than the hard dike, which seems to be a fair estimate, we should 
obtain the period of 33,000 years as that which has elapsed since the 
dike and the inclosing land stood at the same height, which they 
undoubtedly did when the dike was formed. A still longer period 
of existence is indicated by the denuding effect of the sea waves. 
Hocks exposed to their action have not only the corroding and 
oxydating influence to wear them away, but are in addi- 
tion subjected to a mechanical mode of destruction. The sea 
undermines the layers of lava, by washing out the volcanic mud and 
rubble beds which underlie them ; consequently pieces split off, roll 
