GEOLOGY. 339 



gists have found no part of the earth whatever, save new volcanoes recently 

 thrown up, which has not been more than once beneath the ocean, so long 

 and at such a depth as to have sedimentary strata thrown down upon it 

 to a depth of many thousands of feet. There is no part of the world, not even 

 the giant chain of the Andes, which seems to have escaped this submersion 

 and precipitation. What, then, can have been the fate of the earlier and 

 more powerful volcanoes of our globe, than in their turn to have sunk under 

 the sea, and had all their irregularities first rasped and removed by the de- 

 structive action of ages of surf and waves gradually creeping over them, and 

 then having them covered under such depth of strata of hard rock, that, 

 when once more lifted up into the atmosphere, no quarrying by man can 

 ever expose their full proportions. When we go back from Chajorra or 

 Rambleta, at present unextinct, and some three quarters of a mile in diame- 

 ter, to the great crater of Teneriffe, eight miles in diameter, and extinct 

 during the human period, or equally from Vesuvius, at present active, and a 

 quarter of a mile in diameter, to Somma, two miles in diameter, and per- 

 fectly quiet ever since Italy has been dry land, we find the older craters 

 to have been the larger; and if they are not very large as compared with 

 those of the moon, it is because their age is, after all, quite in the mod- 

 ern times of geology, and as shown by the shells found in the lower slopes 

 of either volcano to belong to the post-pliocene period. The grand vol- 

 canic circles, then, of the older " primary " and " secondary " days can 

 never be seen by man; but would he form some idea of their mighty propor- 

 tions, when the crust of the earth was thin, its whole interior fluid with heat, 

 and its more volatile materials going off in oceans of vapor, agitating the 

 whole globe, and reacting against the weak exterior with terrific violence, let 

 him look to the surface our surface of the moon, never yet depressed 

 under an ocean, and there, as in a glass held up to us for our instruction, 

 may be seen what must have been the throes endured by the earth, and 

 what the size of volcanic mouths in its early history of fiery ordeal. Many 

 further differences may be found, on close observation, between lunar and 

 terrestrial volcanoes, as dependent on the infinitesimal atmosphere around 

 the former, and the smaller force of gravity acting on them. To assist in 

 investigating the nature of such modifications, we have, happily, in Tene- 

 riffe, specimens of volcanoes which, at the time of their activity, were 

 some of them submarine and some subaerial; and when we look to the 

 smooth slopes of the former, some 12, increasing to 28 in the latter, with 

 extreme roughness of surface, we can hardly but allow that this is a strong 

 approach to the still steeper and more jagged forms in the moon. In short, 

 with its rare atmosphere (22 mercurial inches), so dry that the rocks do not 

 disintegrate, vegetation does not spread, and a slow change of color is all 

 that marks the lapse of successive centuries, the little volcanic world of the 

 great crater of Tenerifle, raised high above the clouds, is a most important 

 region to be studied and mapped with reference to lunar investigations. To 

 map this region correctly would be a work of years ; and all that I have 

 done is to point out the character of the phenomena visible from two points 

 of the circle, viz., the stations established on Guajara and Alta Yista. The 

 terrestrial part of the problem is thus still only begun, and the greater part 

 remains still to do; while the astronomical portion, or the telescopic part, 

 will have more and more work prepared for it, as often as any character- 

 istics of form are made out by theory or earthly analogy as necessary to 

 volcanic action. Amongst them may be already placed the dynamic waves 



