328 TRAVELS AMONGST THE GREAT ANDES, chap, xviii. 



collected ; and that at Riobamba, upon paj)er similarly exposed, 

 about as much fell per square foot as upon Chimborazo. 



From these data I have calculated the minimum quantity 

 which must have been ejected upon this occasion. Drawing two 

 lines radiating from Cotopaxi, one leading to Riobamba, and the 

 other to an equal distance west of the fifth camp (within which 

 limits it is certain that the dust fell), and estimating that from our 

 cam|) to Ambato only one-eighth of an ounce fell on each square 

 foot, and that from Ambato to the A^olcano four ounces fell on 

 each square foot, I find that, at the least, two 7nillions of tons must 

 have been ejected during this eruption.^ The quantity is under- 

 estimated in several ways. The amount is ignored that was 

 carried beyond the limits which have been indicated, though it fell 

 over many hundreds of square miles. ^ The quantity only is taken 

 into account which was actually found upon the tent — not that 

 which had fallen from it, nor that which had still to descend ; and 

 from Ambato northwards a fall of fifteen minutes only is reckoned, 

 although it continued to settle for several hours. ^ 



I have found it interesting to compare the dust deposited upon 

 Chimborazo with that which fell upon our tent when we were 

 encamped on the summit of Cotopaxi (see p. 153), which was 



^ This amount is equal to a column of solid lava (2*65 spec, grav.) 38 feet square 

 and 18,600 feet high. 



2 And on ships upon the Pacific Ocean. 



3 As pure and undeflled Volcanic Dusts can seldom be procured, and are 

 desiderata with students, I have placed the collections mentioned below in the 

 hands of Mr. J. R. Gregory, 88 Charlotte Street, Fitzroy Square, from whom 

 samples can be obtained. 



1. Dust which fell at Quito (after an aerial voyage of 34 miles) from the great 

 eruption of Cotopaxi in June, 1877 (see p. 125). 2. Dust which fell on the tent on 

 Chimborazo (after an aerial voyage of 64 miles) from the eruption of Cotopaxi, 

 July 3, 1880. 3. Lapilli and dust from the apex of the cone of Cotopaxi (19,500 

 feet). 4. Granular dust from sheltered places on Cotopaxi (see p. 141). 5. Lapilli 

 from 15,000 feet on Cotopaxi. 6. Pumice lapilli from Ambato (see p. 94). 7. Fine 

 dust from the ten-inch bed at Machachl (see p. 104). 8. Fine pumice-dust from the 

 lowest bed at Machachi (see p. 104). 



