1849.] LYELL ON THE STRUCTURE OF VOLCANOS. 221 



p. 216) was left having ten fathoms water over it. This shoal Captain 

 Graves surveyed, and the soundings were found to deepen in all di- 

 rections, demonstrating the existence of a submarine conical eminence. 

 Lieut. Leycester was also told of houses seen at the bottom of the sea 

 on the east of Thera, near the site assigned for the ancient Eleusis ; 

 and a similar statement was made of ruins under water, at the base of 

 the steep cliffs of Therasia ; but as on this coast inside the gulf the 

 water deepens very suddenly from the base of the cliffs, an earthquake 

 may have thrown down some buildings into the sea. It is therefore 

 unsafe to draw any positive conclusion in favour of subsidence from 

 such data. 



When we reflect on the oscillations of land which have occurred 

 within the last eighteen centuries, on the site of the Temple of Serapis 

 near Naples, we may well imagine much greater movements of 1000 

 or 2000 feet to have happened in the course of the geological period 

 during which Santorin may have been exposed to denudation. 



I may observe however, that if a general upward movement should 

 now recommence in this archipelago, so that the crater should emerge 

 at the rate of a few feet or yards in a century, the waves would have 

 power to tear down the rim where it is now perfect at a slight depth 

 under water, namely between Therasia and Aspronisi and betweea 

 the latter island and Thera. The same force which is now denuding 

 the chffs of those islands would readily undermine rocks of diversified 

 and partly incoherent composition, during a continual change of level 

 from century to century. The effects of this slow waste would appear 

 in the form of wide breaches in the outer wall or ring of volcanic 

 rocks, so that the condition of Santorin would approach much more 

 nearly than now to the broken basaltic escarpments of St. Jago and 

 Mauritius, as described by Mr. Darwin. 



There has been som« controversy as to whether the fundamental 

 argillaceous schist seen in the south-eastern part of Thera, or the main 

 island, crops out also in Therasia ; but if so, it would not affect the 

 theory of denudation above proposed ; for we must conclude with 

 Mr. E. Forbes that the original volcano of Santorin was formed in 

 the bed of the Mediterranean, on which the limestone mass and ar- 

 gillaceous schist of Mount St. Elias, now 1887 feet above the sea, 

 formed a submarine mountain, against which the south-eastern base 

 of the great cone abutted. It is therefore very possible, though we 

 have as yet no certain data for the fact, that the same pre-existing in- 

 equalities of the sea-bottom may cause similar ancient rocks to crop 

 out in a part of Therasia. 



Island of St. Paul. 



The volcanic island of St. Paul, situated in the midst of the Indian 

 Ocean, lat. 38° 44' south, long. IT 37' east, and surveyed in 1842 by 

 Captain Blackwood, R.N., may serve in some degree to aid us in con- 

 ceiving how such an archipelago as that of Santorin may have been 

 formed (see figs. 7, 8, 9). In that portion of the volcano, probably a 

 very insignificant part of the whole, whether in height or area, which 

 at present emerges above the level of the wide ocean, we have a crater 



