40 Evolution and Distribution of Fishes 



leagues around people actually deserted their homes, fearing 

 lest their roofs might be crushed in. At Leon, more than 

 a hundred miles away, the dust lay several inches deep, 

 and it was carried to Jamaica, Vera Cruz, and Santa Fe de 

 Bogota, over an area of 1500 miles in diameter." 



When such fine dust falls on the surface of a swamp, 

 a lake, or the sea, it might be supposed that the whole would 

 be swept away, and mixed with materials of erosion. But 

 Judd {ig; 72) has well observed: "Everyone is familiar 

 with the fact that pumice floats upon water; this it does not 

 because it is a material specifically lighter than water, but 

 because cavities filled with air make up a great part of its 

 bulk. If we pulverize pumice, we find the powder sinks 

 readily in water." And later (p. 74) after speaking of 

 oceanic volcanic debris — pumice, dust, etc., — he says: "these 

 particles of volcanic dust and fragments of pumice, by their 

 disintegration give rise to a clayey material, and the oxida- 

 tion of the magnetite, which all lavas contain, communicates 

 to the mass a reddish tint. This appears to be the true 

 origin of those masses of "red clay" which, according to 

 recent researches, are found to cover all the deeper parts 

 of the ocean." 



But it may well be suggested that equally abundant dust 

 deposits have taken place on land and in lacustrine or in 

 swamp areas, where they could rapidly entomb and preserve 

 even delicate organisms. Many evident examples of this 

 will be referred to later. 



The quantity of dust carried and the extent of area 

 covered have both been fairly accurately estimated. In 

 addition to statements made above, the following are help- 

 ful. "In 1882 at Vesuvius the ashes not only fell thickly on 

 the villages round the base of the mountain, but travelled 

 as far as Ascoli, fifty-six miles distant on one side, and 

 Casano, one hundred and five miles on the other." (Geikie 

 3rd edit. p. 213). In the eruption of CotopaxI on the 26th 

 of June 1877 the same author says (p. 213) : "At Guayaquil 

 on the coast, one hundred and fifty miles distant, the shower 

 of ashes continued till the first of July (i.e. five days). Dr. 

 Wolff collected the ashes daily, and estimated that at that 

 place there fell three hundred and fifteen kilogrammes on 



