26 
THE GEOLOGIST. 
phyiy, but, having been thoroughly cracked or jointed in one given 
direction by cooling, after having been heated by contiguity with the 
erupted mass, it has become liable to split into blocks formed by the 
transverse sections of the joints themselves; and these blocks having 
been removed by the action of the waves, which cover the whole reef 
at high water, the porphyry has become exposed to view, and being 
harder than the granite has resisted the force of the water, while the 
granite has been, not exactly worn, but gradually split in such a 
manner as now to lie in ledges against the porphyritic ridge, present- 
ing the curious and unusual appearance which I have attempted to 
describe. This undoubted action of the elements upon the granite in 
other parts of the island has produced some curious results ; but I 
fear the length to which my remarks have already led me will scarcely 
allow of my doing more than briefly to advert to them. At Peninnis 
Head, at Giant's Castle, at Old Town Forth, at the Pulpit, and 
Clapper Rocks, and at various other places in St. Mary's, admirable 
examples occur of almost columnar structure brought about by the 
wearing away of the granite in the direction of the vertical or trans- 
verse joints. Some of these are on a most magnificent scale. The 
Pulpit Rock affords an example of one large mass of granite, estimated 
at 40 feet long, poised in a projecting position like the sounding- 
Ligii. 4. — Granite Blocks at Porth HiUick. 
board of a pulpit, and maintained in its place by a large mass of 
disk-like rock at the base, and effected entirely by the operation of 
natural causes. At Forth Hellick — the reputed spot at which the 
