904 



THE WONDERS OF GEOLOGY. Vol. IT. 



tumefactions, varying from the thickness and delicacy of a soap-bubble, 

 to the size of caverns twenty or thirty feet wide. These caverns, which 

 extend in every direction, form, beneath the surface of the island, sub- 

 terranean channels, through which the overflowing lava makes its way, 

 and are often covered by a hollow arch, which yields at once to the 

 tread. Their interior contains the most interesting incrustations of 

 sublimed minerals, with crystalline forms, the perfection of which can 

 hardly be appreciated without the aid of a microscope, and so delicate 

 as scarcely to bear the breath. Mounds of sulphur, more extensive 

 than those of Solfatara, are deposited around the southern plane of the 

 crater. 



" On the western flank of the crater above described, the appearances 

 render it probable that the former surface of the incandescent matter 

 was 300 feet higher up than it is at present j and that the opening of 

 the crater of Mouna B,oa, which is now 8,000 feet above, diverted the 

 course of the intense subterranean heat from that of Kirauea, or at 

 least diminished its intensity. It seems, also, that the incandescent 

 matter of the interior of the crater became refrigerated and solidified 

 in the mighty caldron ; and that after a lapse of time the base on 

 which it stood gave way, under the renewed agency of subterranean 

 heat, when the mass cracked and slipped. A large mass of the solidi- 

 fied lava appears to have fallen again into the abyss, and been re- 

 molten : while a part remained lodged against the sides of the caldron, 

 and is now seen as a rock two hundred feet high, consisting of basalt, 

 trachyte, and lava of several varieties. Between the scoria ceoas lava 

 approaching to slag, which lies uppermost, and the close-grained basalt 

 which forms the loivest portion of the rock, the transition is so gradual, 

 that it is impossible to assign the spot where bascdt ceases, and lava 

 begins. The words, basalt, trachyte, and lava, serve, therefore, only to 

 distinguish the upper from the lower part of a stream of molten 

 matter." — Strzeleckis New South Wales. 



B. Page 862. — Logan or rocking stones. — In that most successful 

 of all the attempts to clothe science in the garb of fancy, — the delight- 

 ful volume called " Philosophy in Sport made Science in Earnest," — 

 there is an interesting account of the rocking-stoncs of Cornwall, which 

 the antiquaries of the last century claimed as Druidical monuments ; 

 but which have originated in the natural causes explained in the fol- 

 lowing description of the celebrated Logan or logging- stone, near the 

 Land's End : — 



" The foundation of this part of the coast of Cornwall is a stupen- 

 dous group of granite rocks, which rise in pyramidal clusters to a 

 great altitude, and overhang the sea. The celebrated Logan-stone is 

 an immense block, weighing above sixty tons. The surface in contact 

 with the under rock is of very small extent, and the whole mass is so 

 nicely balanced, that, notwithstanding its magnitude, the strength of 

 a single man applied to its under edge is sufficient to make it oscillate. 

 It is the nature of granite to disintegrate into rhomboidaJ and tabular 



