Prof. J. Milne — On the Form of Vokanos. 339 



sucli an action is to make a mountain steeper near tlie top than lower 

 down, an appearance which, will be again referred to. 



If we wish to discover an explanation for the differences which 

 exist between the characters of the volcanos of two countries like 

 Iceland and Japan, it is not to the effects of denudation that we ought 

 to look, for denudation in the case of the volcanos of these two 

 countries, which are either now in action or else have been so within 

 periods which are almost historical, might, I think, be taken as being 

 equivalent to each other. We must rather look to the nature and 

 the products of the eruptions. In Iceland I suspect that the out- 

 bursts have been more paroxysmal, whilst the number and extent of 

 lava flows are perhaps unrivalled. These causes haA'^e given a ragged 

 wildness to the country and its mountains. In Japan, however, 

 lava streams, although existing, are by no means numerous, and the 

 mountains, which far exceed the height of those in Iceland, must, 

 generally speaking, have grown gently upwards like a heap formed 

 by sand falling through a funnel upon a level floor. In consequence 

 of this it will be shown that they have in many cases assumed a 

 form as symmetrical as any which would be expected in an en- 

 gineering earthwork. 



Slopes of Volcanos. — If we look at the cuttings along a railway, 

 we shall see that their slope varies with the material through which 

 they are run. When the material is loose and friable, the slope is 

 more gentle than when it is tenacious and has much friction amongst 

 its particles ; for example : 



For compound eartli the natural slope is 50°. 



„ gravel „ „ „ 40°. 



„ dry sand „ „ „ 38°. 



jj SBiQCL 9^ jj jj ...„ ZZ . 



,, wet clay ,, „ ,, 16°. 



When climbing up the sides of volcanos I have invariably observed 

 that the materials resting upon any particular slope are uniform in 

 size ; and if we take a piece of lava or lapilli which is larger than 

 the other particles, and throw it upon such a slope, we see it com- 

 mence to roll rather than remain at rest. From this it appears that 

 we ought generally to find the large pieces of material which are 

 ejected from a volcano upon the more gentle slopes near to the base 

 of the mountain. The reason of this evidently lies in the fact that 

 friction depends on surface, and weight depends on volume, and 

 that as bodies are made smaller and smaller the ratio of their surface 

 to their volume increases more and more. 



Should we be desirous of making any investigation respecting the 

 slopes upon the face of a long escarpment, or the face of a broad 

 chain of mountains, the theory and rules given by Eankine in his 

 Investigations about Earthworks might be applied. In a volcano 

 or solitary mountain, such as those which are now referred to, the 

 conditions would be somewhat different, as we have to consider the 

 stability of a mass of material with a variable radius, whilst in the 

 former case the radius might be regarded as being infinitely long. 

 As there does not appear to be any formula given in books on 

 engineering, respecting the slope or form which would be assumed 



