VOLCANOES 347 
Hawaiian Islands, the lava that cools in this fissure 
forms a dyke (Fig. 210). When the crater is plugged, 
the attempt of the lava to reach the surface often 
breaks the strata, and thus also dykes are intruded 
into fissures, giving temporary relief to the strain that 
“Fre. 210. 
Dykes of black diabase crossing lighter granite. 
is forcing the lava upward. Near every volcano, deep 
down below the surface, there are numerous dykes 
formed, each representing either a successful or an 
ineffectual effort on the part of the lava to escape. 
Such intruded dykes are found far from volcanoes, 
where denudation has laid bare the formerly buried 
strata. 
