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. 



Veins of Lava. Punto di 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 



