316 



THE GEGLOGIST. 



flat plains — by the uprise and bursting of a vast dome or babble ; 

 and sucli was called a "crater of elevation". 



So commonly in England have we been accustomed to regard the 

 great mountain-mass of every volcano as successively and con- 

 tinuously built up by the lavas and scoriae rejected from its orifice, 

 that we observe with astonishment the prevalence to which the 

 " crater of elevation" doctrine, by being favoured by Humboldt and 

 Yon Buch, and some other great authorities, has attained. 



Sir Charles Lyell early observed the danger of allowing this erro- 

 neous doctrine to hold its way, or to spread, and from the first 

 edition to the ninth of his " Principles of Geology," he opposed its 

 tenets ; and, especially after his retui^n from Madeira, in 1852, he 

 controverted its essential point by some well-selected instances of 

 stony lavas consolidated at steep angles. 



Apparently feeling, however, the necessity of grappling with and 

 thoroughly exploding this patronized falacy. Sir Charles, in 1857, 

 visited Etna, and obtained conclusive examples of the capability of 

 lavas forming stony masses on slopes of not less than from 40 to 47 

 degrees, an account of which he laid before the Royal Society. In 

 October, 1858, Sir Charles again visited Etna, and obtained further 

 confirmatory proofs, which have been engrafted on his original 

 memoir, and appear in the last part, recently issued, of the Philo- 

 sophical Transactions. 



As the " crater of elevation" theory is built entirely upon the 

 assumption that lavas will not consolidate on steep slopes, it is evi- 

 dent that, by attacking and demolishing the foundation, the super- 

 structure must fall, and thus the chief object of Sir Charles Lyell's 

 two visits to Etna was to collect evidence of the consolidation of 

 lavas which flowed down declivities at high angles into tabular stony 

 masses. 



The first example given is the highly-inclined stony lava of Aci 

 Reale. 



The town of Aci Reale stands on the top of a clifi* in which a 

 platform, elevated at some points more than 650 feet above the sea, 

 ends abruptly. The slope of this platform is usually about three 

 or four degrees, and is prolonged two or three miles inland, while 

 the clifl* between the town and the sea consists of irregnilar 

 precipitous terraces, and exposes on its face the truncated edges 

 of several lava-currents, which were noticed by the Canon Recu- 

 pero in his " Storia Naturale dell' Etna," to which the traveller 

 Brydone called attention in England in his " Letter on the Two 

 Sicilies." 



In the face of this steep escarpment, facing the sea, an indentation, 

 near the Bastione del Tocco, afibrds a longitudinal section of one of 

 these lava-currents dipping east at from 23 to 27 degrees, and pre- 

 senting all the usual characters of an upper and under scoriee with 

 a central stony mass. 



^ This case is supported by another still more remarkable and de- 

 cisive instance, in a branch of the great stream of 1689 which 



