26 



THE GEOLOGIST. 



phjay, but, having been thoroughly cracked or jointed iu 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 gTanite 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- 



Lign. 4.— Granite Blocks at Forth Hellick. 



board of a pulpit, and maintained in its place by a large mass of 

 disk-like rock at the base, and eflected entirely by the operation of 

 natural causes. At Forth Hellick— the reputed spot at which the 



