REVIEW OF VOLCANIC ACTION. 361 
will consequently be the resulting balance between the two operations, 
summit eruptions and lateral eruptions (supposing, of course, that the 
action does not lose any of its force); and it will be nearly a constant 
quantity, rendered variable only by the different character or strength 
of the rocks, the depth of solid material, and the presence or absence 
of open cavities below. This is one source of the dikes or fissures 
about volcanoes. 
Action of unusual violence is another source, and it sometimes splits 
down the cone from its summit to its base. 
Whichever way dikes are formed, their influence on the form of a 
mountain cone will depend on the portion of the mountain they inter- 
sect. If they take the line of radii, commencing at the centre, and 
extend part way to the base, they necessarily cause an elevation, and 
increase the inclination ; but if they only intersect the lower slopes, 
they diminish the inclination, and, at the same time, enlarge the dia- 
meter of the mountain. If they extend from summit to base, of equal 
width, or run transversely to the slopes, they can have little effect 
except in enlarging the mountain. ‘These dikes, as observed in the 
Pacific, are remarkably uniform in width from top to bottom, even 
when exposed for a thousand feet in height. No instance of enlarge- 
ment above was observed, and they were seldom much larger below. 
Neither were examples met with of tiltings or dislocations by these 
dikes, beyond faultings of a few feet.* 
An exemplification of this action of dikes is presented us in Mount 
Loa. The slopes, as shown on page 170, have been extended twenty- 
five miles beyond Kilauea, and reduced to an average angle of 1° 28’, 
while above Kilauea, the angle is 6° 45’. And this process of extend- 
ing the slopes is now going on; for the eruption of 1840 had this 
effect. Indeed all the eruptions of Kilauea, for a long period, have 
taken place towards the sea, or beneath it, and have resulted in 
tify the sides, and enable the mountain again, perhaps, to pour forth lavas from the 
summit. 
* This non-disturbance of the rocks by volcanic dikes is dwelt upon by M, Constant 
Prevost.— Bulletin de la Soc. Geol. de France, xi. 196. 
Darwin also remarks on the great number, height, and even width of the dikes of St. 
Helena; and observes that one exposed to view for a height of 1260 feet, was but four 
inches narrower above. An instance of remarkable disturbance is mentioned, and the 
same was seen by the writer; but he adds that dislocations on so grand a scale are ex- 
tremely rare in volcanic districts,— Vole. Islands, pp. 77, 78. 
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