Ch. VII.] 
DIKES ON ETNA. 
91 
blue basalt with olivine. They vary in breadth from two to 
twenty feet and upwards, and usually project from the face of 
the cliffs, as represented - in the annexed drawing (No. 19). 
They consist of harder materials than the strata which they 
traverse, and therefore waste away less rapidly under the in- 
fluence of that repeated congelation and thawing to which the 
rocks in this zone of Etna are exposed. The dikes are, for 
the most part, vertical, but sometimes they run in a tortuous 
course through the tuffs and breccias, as represented in dia- 
gram, No. 20. In the escarpment of Somma where, as we 
No. 20. 
Feins of Lava. Punto cli Guimento. 
before observed, similar walls of lava cut through alternating 
beds of sand and scoriae, a coating of coal-black rock, approach- 
ing in its nature and appearance to pitch-stone, is seen at the 
contact of the dike with the intersected beds. I did not ob- 
serve such parting layers at the junction of the Etnean dikes 
which I examined, but they may perhaps be discoverable. 
The geographical position of these dikes is most interesting, 
as they occur in that zone of the mountain where lateral erup- 
tions are frequent ; whereas, in the valley of Calanna, which is 
below that parallel, and in a region where lateral eruptions are 
extremely rare, scarcely any dikes are seen, and none whatever 
still lower in the valley of St. Giacomo. This is precisely 
what we should have expected, if we consider the vertical 
fissures now filled with rock to have been the feeders of lateral 
