VOLCANIC DUST AND PUMICE IN MARINE DEPOSITS. 491 



the sea or in places on the land, whence it would be washed by the rain to the 

 ocean, has amounted to somewhere near a cubic mile per annum. Although the 

 Javanese archipelago appears to be, for its area, the place where the largest amount 

 of vesicular or finely divided lava is now produced, we cannot, on the review of 

 the facts, assume that more than from one-fifth to one-tenth of the contributions of 

 those materials come from this limited field. Therefore, on the basis of this very 

 rough reckoning, we may perhaps estimate the annual contribution to the seas of 

 these rocks which may float in the water at somewhere near five cubic miles. 



The Mississippi river each year discharges into the sea about one-twentieth of a 

 cubic mile of suspended or dissolved mineral matter. It is doubtful if the aggregate 

 discharge of the rivers of the world amounts to more than thirty times that of the 

 Mississippi, so that the importation of sedimentary materials into, the sea by the 

 rain water may be in quantity much less than that contributed directly by the vol- 

 canoes. As yet the value of coastal erosion is not even approximately known, but 

 assuming that the eastern coast of the United States affords a fair basis for such 

 measurement, it seems likely that the volcanic ejections which fall upon the sea or 

 are quickly washed into it equal, if they do not exceed, the coastal detritus. 



The distribution of the vesicular fragmentary lava which may float upon the sea 

 is evidently wider than that of the ordinary detritus from the land. In general, 

 the range of the carriage of the fragments depends on the specific gravity of the 

 bits. It is easily seen that there is a great diversity in this feature. As regards the 

 dust, there is also a variation due to the size of the fragments. These may be so 

 small that, as in the case of those formed during the eruption of Krakatoa, the rate 

 of fall even through the air may be very slow. These diversities in the rate of 

 descent probably result in the deposition of a great part of the dust and pumice on 

 the sea-bottom at no great distance from the vent. A large portion of these ma- 

 terials evidently journey far and normally find their place of rest on the seashore 

 in the well known manner of driftwood. As evidence of a certain though limited 

 value as to the truth of this proposition the following facts may be noted : 



Along the eastern and southern coasts of Florida the writer in 1887 noted the 

 occurrence on the beach of numerous fragments of volcanic pumice. The quantity 

 was so considerable that a number of observations showed that each square yard 

 of the surface would on close inspection reveal a number of bits nearly all of which 

 were evidently breaking up under the blows of the waves into unrecognizable 

 powder. A further search of the Atlantic coast as far north as Eastport, Maine, 

 has shown the presence of this material, though in lesser quantities. An extended 

 correspondence has indicated like occurrences of pumice on the Pacific coast of 

 the United States. 



Although there is danger that inexperienced observers may mistake the puma- 

 ceous ash which is formed about the grate bars of the boiler fires of steamships or 

 in other similar conditions, it is not difficult to discriminate the natural from the 

 artificial product, at first by the included minerals and after the eye is well trained 

 by the macroscopic aspect of the material. 



Some of the bits of pumice which has recently come upon the Atlantic coast of 

 the United States somewhat closely resembles that which was thrown out by 

 Krakatoa in August, 1883. Witliout making too much of this likeness, it may be 

 noted that there is no other more likely source of origin of these fragments. 



The main points of this paper pertain to the question as to the amount of the 

 material from volcanoes which may float in the sea (an amount which though not 



