Several investigators have tackled the problem of the main causes that produce the slopes of volcanic cones, especially with a view to explaining the characteristic concave profiles of strato-volcanoes *). A satisfactory result has not been arrived at, however. This became evident to the present author while studying the submarine slopes of volcanoes in the East Indies. A number of submarine sections of isolated volcanic piles were constructed from the echo-soundings of the Snellius Expedition and from the data contained in the fair sheets of the Hydrographical Survey. These sections combined with the corresponding subaerial profiles will be reproduced in the Scientific Results of “the Snellius Expedition, Volume V: Geology, Part 1: Geological Interpretation of the Bathymetrical Results”, together with a discussion of their shapes and the mode of their formation. An explanation of the wet part of the slope is not possible, however, until we understand the agents influencing the dry part. But as we said, this subject has not been adequately treated. An attempt had therefore first to be made to analyse the factors that play a part in the production of subaerial slopes of volcanoes. In order to test the validity of the deductions an experimental investigation was undertaken that will be described below. These experiments were carried out in the laboratory for experimental geology in the Leyden geological institute.