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



3. Fine gravel mixed with sand. — This fell to form a heap 26 inches high. 

 Hound the base a slight curvature was observed. 



4. A mass of gravel mixed with sand was formed by throwing it against a board, 

 on which a sheet of paper had been stretched. Here also a slight curvature was 

 obtained, which was marked off upon the paper. 



Taking these experiments as a whole, it will be observed that I 

 did not obtain much evidence in favour of my views. In experi- 

 ments number 3 and 4, where slight curvatures were obtained, it 

 must be noticed that I was using a material the particles of which 

 were of different sizes. Had the experiments been upon a larger 

 scale, I think the results might have been more satisfactory. Also, 

 if the sand and gravel, instead of falling in a column, had fallen as 

 a rain upon the heap, and the heap had subsequently been left some 

 months to settle, the curvatures would in all probability have been 

 more decided. 



By allowing water to fall for some time as a light shower over 

 the heaps, I endeavoured to imitate the effects of denudation. Although 

 many of the showers were infinitely greater in proportion to the size 

 of my heaps than the showers of rain which fall upon a mountain are 

 .to the mountain and its materials, the dragging of materials to the 

 base was not sufficient to produce any observable curvature. This 

 would seem to show that it is not denudation of a kind like this 

 to which we must turn in order to find an explanation for the 

 curvatures of volcanos. 



In looking at any large heap of materials, the component parts of 

 which are of different sizes, a curvature will, I think, be always 

 observed. 



A short time ago I observed this upon a number of heaps of gravel, 

 which, for purposes of measurement, had been piled in large rect- 

 angular cases. These cases had subsequently been removed, and 

 the heaps left to themselves. First, the sides had fallen downwards 

 and outwards, the larger stones rolling the farthest, and a curvature 

 had been produced. Since the first general form had been assumed, 

 a similar action had been slowly going on under the combined 

 influence of gravity, and, to a slight extent, the weather. 



These heaps were about 3 feet high. Their curvature was best 

 observed by standing at a distance, and then stooping until you 

 brought your eye on a level with the heap. 



As the result of these experiments and observations, I would 

 classify the principal causes which tend to affect the form of a 

 volcano under the following three heads : — 



1st. Causes tending to give a volcano its characteristic curvature. 



a. The tendency of a self-supporting heap under the influence of its own weight 

 to spread outwards at the base. This would tend to give a logarithmic curvature. 



b. The tendency during the building up of a mountain of the larger particles to 

 roll farther than the smaller ones. This will also depend on the specific gravity, the 

 porosity, etc. , of the particles thrown out during an eruption. We may regard this 

 action as extremely rapid denudation carried on by gravity. 



c. Denudation by weather and gravity. This tends to efface a mountain by 

 digging away materials from the top and making its steepness less and less. At the 

 bottom of the mountain, where denudation is also going on but at a slower rate than 

 it takes place near the top, these materials are deposited and spread out. 



