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Transactions. — Geology, 



was extremely weak at first, scattering the material to such a 

 short distance from the centre of eruption that a very minute 

 cone (as a a') was formed, and that the force gradually and 



I7S/1 



a a' 



uniformly increased as the eruption progressed, reaching its 

 maximum intensity as the eruption began to die out. Such 

 a state of things is not suggested, and would be difficult to 

 imagine as actually occurring. Indeed, if anything, the first 

 eruption would probably be of greater intensity and violence 

 than those succeeding it, clearing away obstructions and 

 forming a vent by which subsequent ejection of material would 

 be more easy, and attended by less violent explosions. Of 

 course, after the slope of the sides reached about 35° the 

 material would no longer remain in a state of repose where it 

 was laid down, but would slide and roll downwards, so that 

 the angle of slope would then become constant, or slightly 

 decreasing. A section of a normal volcano would therefore 

 present something of the form shown in fig. 2. 



Fiy2 



a, b, c. The cone when its sides have attained an angle of 35°, and before 

 the materials have begun to roll to any extent, c', d, e. The cone 

 after this angle is passed. The lines show the cone at progressive 

 stages in its growth, and represent the beds sloping at a constantly- 

 increasing angle. 



3. Lava-cones. — Eangitoto is an example of this class. 

 Only its upper part consists of scoriae and ash, the whole of 

 the lower slopes being formed of lava- streams. It rises at a 

 low angle (from 5° to 10^) to the foot of the peaks or scoria- 

 cones forming its summit. These dip at the usual angle of 

 30= to 35°. 



Having described the three classes into which these 

 volcanoes may be divided, we may now pass on to consider 

 the material of which they are composed. This is in every 

 case a basalt very rich in olivine, and showing great similarity 

 at all the points of eruption. The scoriae are of all sizes, 

 from dust and small cinder-like fragments to large masses 

 weighing several tons. The smaller fragments are generally 

 highly vesicular, so much so in some cases that, though a 

 heavy basic rock, the enclosed air enables it to float on water. 

 Bombs and curiously twisted and contorted fragments of lava 



