560 PROCEEDINGS OF THE SAINT LOUIS MEETING 



the gorge of the Riviere Blanche by the material brought from the crater by the 

 numerous exploding dust clouds which have been the distinguishing feature of the 

 numberless small eruptions that have taken place from the volcano since May 8, 



1902. The lower portion of the gorge has been entirely obliterated and the adjoin- 

 ing plateau elevated, while the upper and deeper portion near the center has been 

 almost filled by ejecta. The dust flows are the material left behind by the dust- 

 laden clouds of steam. The exploding clouds of steam were so overloaded with 

 dust and larger fragments of comminuted lava that they flowed down the slope of 

 the mountain and the gorge like a fluid propelled at a high velocity by the hori- 

 zontal or partly downward component of the force of the explosion. Many large 

 fragments of solidified lava were carried down the gorge by these clouds. Such 

 blocks, 10 to 15 feet in diameter, were not uncommon. 



SOME EROSION PHENOMENA OBSERVED ON THE ISLANDS OF SAINT VINCENT 



AND MARTINIQUE IN 190% AND 1903 



BY EDMUND OTIS HOVEY 



The deposition of a coating of new material, varying from a few inches to many 

 feet in thickness, over an area embracing about 50 square miles on the island of 

 Saint Vincent and nearly as much territory on the island of Martinique, gave an 

 instructive opportunity for the study of the development of erosion^forms under 

 the influence of tropical rains. The new layer was thicker and more evenly dis- 

 tributed around the crater of the Soufriere of Saint Vincent, and hence the devel- 

 opment of erosion features was more satisfactory there than it was in the vicinity 

 of the crater of mont Pelee. On the hill slopes the extensive development of den- 

 dritic drainage was very striking. Along the crests of the ridges on the Soufriere 

 the fine dust of the eruptions of May, 1902, was turned into a cement-like mud of 

 considerable tenacity, which retained its place, and was covered over by the 

 heavier material thrown out by the eruptions of September and October, 1902, 

 and March, 1903. In the valleys the permanent and periodical rivers were loaded 

 with the new ash to such an extent as to form viscous streams, which, however, 

 had great powers of erosion on account of the steep slope of the declivities down 

 which they flowed. The bottoms and sides of the gorges were deeply grooved by 

 the sand carried down in this manner by the flowing waters. 



During the great eruptions the ejected material was drifted into large beds in 

 the gorges extending radiall} T down the Soufriere. The massing of material was 

 most important in the gorge of the Wallibou river on the west and in that of the 

 Rabaka river on the east side of the island. In these gorges the bed of new ma- 

 terial reached a thickness of from 60 to 100 feet. This enormous amount of 

 material was almost entirely washed out of the gorges during the first rainy sea- 

 son following the eruptions of 1902. Not less than 150,000,000 cubic feet of ashes 

 have been washed out of the Wallibou gorge itself, without taking into account 

 the thousands of cubic. yards of fresh ash removed from the watershed of the river 

 during the same period. All of this material was, of course, transported directly 

 to the ocean. The shoreline of the mouth of the Wallibou was pushed out not 

 less than 100 yards by the deposition of sediment between May, 1902, and March, 



1903. Similar extension of the coastline occurred at the mouth of other rivers 

 along the west side, but in general the shore is too steep along that side of the 

 island for a visible accumulation of debris. 



