Prof. John Milne — On the Form of Volcanos. 509 



It roust here be noticed that on a volcano, and especially near its 

 summit, material is seldom to be found lying at an angle which is 

 less than its angle of repose. Of this fact I have convinced myself 

 that it is the case upon the slopes of many volcanos ; 1st, by ob- 

 serving that the materials on any particular slope were uniform in 

 size ; and, 2nd, that if upon such a slope I dropped a piece of 

 material larger than those of which the slope was made, it com- 

 menced to roll towards a lower level. 



If this be granted, then the levelling action of denudation cannot 

 have played a very important part in giving the form to a volcano ; 

 otherwise we should expect to find materials lying at angles less 

 than that which a heap of them would naturally exhibit. However, 

 denudation must have had some effect, and as this effect will pro- 

 bably have been nearly the same on all sides of the mountain, it 

 will not materially affect its regularity. 



2nd. Causes principally affecting the regularity of a mountain. 

 About these I have written before. Briefly they are as follows : — 



1. Position of the crater. If this is central, we should expect the 

 mountain to be regular. 



2. Lateral and parasitic craters will tend to destroy the regularity. 



3. Direction of the wind during an eruption. 



4. Nature of the eruptions. If these have been paroxysmal, and 

 have blown away portions of a crater, — if lava has flowed on one 

 side more than on another, etc., we have causes tending to destroy 

 regularity. 



3rd. Causes partially affecting the form of a volcano, but prin- 

 cipally its height. 



The movement of the strata upon which a volcano is built. 



As has been pointed out by Mr. Mallet, the weight of a volcano 

 may tend to crush the strata on which it rests so as to form a saucer- 

 shaped depression. Round the bottom of the mountain the ground 

 may be caused to rise. 



Evisceration or honeycombing beneath a mountain may also cause 

 a depression to take place. 



I would also suggest that because it is probable that during the 

 period of activity, which is a length of time to be measured by 

 many years, the rocks beneath the protective covering which has 

 been built above them must become extremely hot, and the mountain 

 will have an augmented height due to the expansion of the beds 

 upon which it rests. When the activity becomes less intense, con- 

 traction sets in, and a sinking may take place. 



To form any accurate idea of the effect which may be produced 

 by any action of this description is very difficult. If we think of 

 the small amount of heat which passes thi-ough the bricks of a fur- 

 nace, we might suppose that the conduction of heat from the heated 

 nucleus of a volcano into the surrounding rocks would be too small 

 to produce any appreciable effect, whilst if we think of the rapidity 

 with which heat travels through a glass into which we pour hot 

 water, we might arrive at opposite conclusions. The rate at which 



