1884.] Microscopic Structure of some Rocks from Ecuador. 125 



just mentioned ; the most common in the Batavia dust being a very 

 light brown " fluted " or possibly porous glass, with occasional chips 

 of the pumice itself. In short, in the Krakatoa rock-materials, 

 " glass foam " predominates, while in those of Cotopaxi we have a 

 much larger proportion of either chips of ordinary glass or more 

 solid scoria. One -would, therefore, assume, and this would be in 

 accordance with the far greater violence of these eruptions of Kra- 

 katoa, that the molten materials in this volcano had become far more 

 completely saturated with steam at a high pressure before the 

 explosions occurred, which hurled them into the upper regions of the 

 atmosphere. Herr B. O. M. Verbeck, in a very interesting report on 

 Krakatoa, printed in " Nature," vol. xxx, p. 10, states that the steam 

 cloud from Krakatoa on May 20, 1883, must have reached a height of 

 at least 11,000 metres (about 36,000 feet), and during the more 

 violent explosions of August 26 and 27 may very well have reached 

 15 to 20 kilometres (49,213 to 65,618 feet). Cotopaxi, by its frequent 

 " puffings," exhales, as it were, the imprisoned vapours, and thus its 

 ejectments are less uniformly vesicular. It would be worth noticing 

 whether the materials ejected from volcanoes which had erupted after 

 long intervals of repose were more pumiceous than those discharged 

 from vents where there is always more or less of disturbance. 



Chimborazo. 



" Chimborazo has for centuries been known to be a very lofty 

 mountain. Humboldt (" Aspects of Nature," vol. i, p. 96) says that 

 for a long time into the present century it was " everywhere regarded 

 as the highest mountain in the world," and it still ranks as the most 

 elevated in Ecuador. From my mercurial barometer observations on 

 the summit on January 4, 1880, and July 3, 1880, its height appeared 

 to be 20,545 and 20,489 feet respectively. The height I adopt, 

 namely, 20,517 feet, is the mean of these observations. According to 

 Humboldt (" Aspects of Nature," vol. i, p. 57), its height is 21,423 

 feet. 



" The summit of Chimborazo bears north-east from Guayaquil, 

 distant about 91 miles. The valley of the River Chimbo forms the 

 boundary of the mountain, properly speaking, on the western side. 

 This river at the bridge of Guaranda is 8,530 feet above the sea. On 

 the north, the depression (used as a pass) between the mountains 

 Carihuairazo and Chimborazo is the northern boundary of the latter. 

 This is 14,400 feet above the sea. On the south it is bounded by the 

 great sandy plain called the Arenal Grande, the highest point of which 

 is about 14,000 feet above the sea, and is traversed by the track to 

 Quito. This plain is covered with debris from Chimborazo, which 

 in various places has been re-assorted, and now forms stratified 

 deposits. On the east it is scarcely possible to say where the mountain 



