1859.] on the Conical Form of Volcanoes. 127 



ceous crust on the left side ; a removal caused by the annual waste of 

 the steep bank of the ravine produced by the action of rain, and the 

 torrent which flows at the bottom. The ravine intersects alternating 

 beds of tuff, scoria, and lava, slightly inclined to the east, or seawards, 

 being a series of the older products of Etna. This new and steeply 

 inclined lava consists of three parallel layers, an upper fragmentary 

 and scoriaceous mass about 8 feet thick ; a central stony layer, 5 feet 

 thick ; and a lower bed consisting of thin strata of fragmentary scoriae, 

 in all three feet thick, but the bottom part of which is not visible. 

 The compact central portion is a dolerite or trachi-dolerite, containing 

 crystals of felspar with some olivine, and is of the ordinary specific 

 gravity of trap. It is divided by joints, 9 or 10 feet apart, so that 

 among the fragments detached by denudation, and strewed over the 

 sloping bank and bottom of the ravine, are angular masses of huge 

 size, with a fracture like that of many ancient igneous rocks. The 

 normal thickness of this bed of compact dolerite is 5 feet, where it 

 dips at 32° and 35°, but near the top where it first enters the ravine, 

 and where the inclination increases to 45° and 47°, the thickness is 

 reduced to one-half or 2^ feet ; yet when dipping at 47°, it is still not 

 only stony and compact, but there is no breach whatever of continuity 

 in the mass, and not more joints than in the less inclined portion. 

 This branch of the lava of 1689, which has given a new facing to part 

 of the right bank of the Cava Grande, exhibits but slight inequalities 

 on its surface, appearing almost even when contrasted with the main 

 current of the same date, from the surface of which many parallel and 

 longitudinal ridges project prominently, sometimes 40 feet above their 

 base, and with very steep sides sloping at angles of from 35° to 70°. 

 The dip of the main current is between 10° and 16° east. From this 

 and other examples, it is inferred that wherever the slopes are ex- 

 cessive (between 25° and 45°) the surfaces of the cooling lavas are 

 less rugged than where the melted matter has congealed on more level 

 ground. 



Allusion was next made to some lavas which have cascaded over 

 sea-clifFs 500 feet high, between Aci Reale and Santa Tecla. One of 

 these at a place called the Scalazza of Aci Reale, exhibits a longitu- 

 dinal section of a tabular mass of stony rock 20 feet thick, inclined at 

 angles of 23° and 29°, which is connected uninterruptedly with the main 

 body of the same lava resting on the gently sloping platform above, 

 of which the sea-cliff" is the abrupt termination. The above-mentioned 

 highly inclined stony lava is covered as usual by a parallel layer of 

 scoriae (in this case 12 feet thick,) and its base consists of another bed 

 of scoriae of slight thickness. 



Several other sections of modern lavas of Etna, which have not 

 been disturbed in their position since the day of their formation, and 

 which are inclined at angles exceeding 30° were then enumerated. 

 For a detailed account of those, reference was given to a paper by the 

 lecturer, recently published in the Philosophical Transactions (Part 

 2 for 1858, p. 703). Among them is a current, inclined at 36°, occur- 

 ring in the Cava Secca, a deep valley near Zafarana ; and another 



