1849.] 



LYELL ON THE STRUCTURE OF VOLCANOS. 



231 



would be thrown as much out of the perpendicular as the beds they 

 mtersect, when the latter were tilted by subsequent movements. The 

 dikes therefore which have been feeders ought to slope at angles of 



Fig, 10. — Volcanic Dikes. 



between 23° and 27° to the horizon, whereas in the drawing which I 

 made of cliffs in the Val del Bove, I have represented them as nearly 

 all perpendicular. 



Had I seen a dike appearing to blend upwards with a sheet of lava, 

 I should not have inferred any actual connection, unless I could 

 have scaled the cliff, which is unfortunately inaccessible, and, hammer 

 in hand, tested every inch of the junction. But had I thus assured 

 myself of the fact, I should have first inquired whether the dike may 

 not have sent off veins or branches which had penetrated between 

 pre-existing parallel strata. If, however, I abandoned this idea as 

 improbable because a sudden change of direction at right angles could 

 scarcely occur or very rarely in such intrusive veins, I should have 

 speculated on the possibility of such dikes having been filled partly 

 from below and partly from above. After violent eruptions, the 

 flanks of Etna have been fissured, and a bright light emitted from the 

 rents has shown that there was incandescent lava below, although it 

 has sometimes never reached the surface. It is conceivable, therefore, 

 that lava-currents, descending from the higher and more central parte 

 of the cone, might in their way fill up some rents of this kind, the tops 

 of which are often left gaping after eruptions. Such a conjecture would 

 at least relieve me from the extreme embarrassment in which I am 

 placed by M. de Beaumont's hypothesis, for I am not called upon in 

 that case to regard the dikes as the feeders of a series of uniform and 

 parallel beds of lava, with their accompanying strata of intervening 

 lapilli and scorise. The whole might then be imagined to have been 

 poured out or projected from a permanent and powerful central vent, 

 the eruptions being on a grand scale, so as to allow of a considerable 

 degree of uniformity in the spreading of the materials over wide areas, 

 on the sloping side of a great cone inclined at angles between 4 and 

 10 degrees. A steeper inclination may have been afterwards acquired 

 during the distension and injection of the mountain mass. 



