24- GEOLOGICAL MEMOIRS. 



cone which appear to have been most recently convulsed. The cavity 

 at the eastern foot of the cone is 150 feet in diameter and 65 feet in 

 depth, and contains three groups, two at the sides in activity, and 

 one extinct in the centre. These are the first that are met by a 

 person descending from the east, and are the only ones seen in fine 

 weather from the summit of the eastern crater. At a short distance 

 to the right of this cavity is a fissure about 4 inches in breadth, from 

 which issue vapours ; and on the left a single vent occurs in the 

 midst of vegetation, which grows luxuriantly within a yard of the 

 orifice. 



In mounting the cone, two more groups of active vents are reached; 

 and finally at the summit, the most considerable and imposing group 

 of all. It contains nearly forty active vents within a cavity 260 feet 

 in diameter and 65 feet in depth, and exhibits proof of tremendous 

 exertions of force. Cubical masses of rock, upwards of 12 feet in the 

 side, are thrown about in the utmost confusion ; while between their 

 interstices the most sufi'ocating vapours arise. 



Lastly, at the foot of the cone are found two more groups of ex- 

 tinct vents. The total number of active vents is about seventy. 



Vapours also find their way through the loose soil, which consists 

 of ashes, sand, and sulphur: their odour was that of burnt sulphur 

 and of rotten eggs ; from which it is to be presumed that they con- 

 sist of a mixture of the sulphurous and hydrosulphuric acids. The 

 authors next mounted with incredible labour to the summit of the 

 volcano, whose crest is serrated with sharp pyramidal rocks, resem- 

 bling the teeth of a saw. The inner walls near the top consist of 

 detached blocks and rocks of all sizes ; and lower down, of sand and 

 soil with occasional patches of vegetation. The rocks blackened by 

 time, the profound obscurity, and the vast columns of smoke issuing 

 from an abyss 2460 feet in depth, are described as forming a majestic 

 and terrible scene. 



The authors give the following reasons for believing the eastern 

 crater to be the more ancient. It contains no traces of volcanic 

 fumerolles, and its cone of eruption has entirely disappeared; its 

 interior walls are but slightly inclined ; and the ridge which separates 

 the two craters, though gently inclined towards the eastern crater, is 

 cut off" almost perpendicularly towards the western. The trachytic 

 rocks of the eastern crater are covered over with sand and pumice, 

 which have evidently been ejected from the western. The eastern 

 crater burst forth near the summit of the ancient Pichincha, and 

 the western on its side. 



The later eruptions of Pichincha have produced nothing but 

 pumice, that being the only rock visible at the surface. Below the 

 Arenal, the sides of the mountain are covered with vegetation, the 

 surface being composed of soil, sand, and pumice, without any debris 

 which can be attributed to recent convulsions. The few masses of 

 rock which pierce the vegetable crust are probably part of the 

 interior stony structure. Yet the eruptions which caused the present 

 craters must have been tremendous : solid rocks which once formed 

 the summit of Pichincha and the matter thrown from the interior must 



