Mineralogy and Geology. 417 



element of the bones of the vertebrate animals, and carbonate of lime 

 the chief material is the skeletons of the molluscous animals and zoo- 

 phytes, silica is almost peculiar to these minute races of infusoria. 

 Some of the marls in which these silicious animalcules are found 

 contain a large admixture of pumice, giving it the character of vol- 

 canic tufa. 



4. Exploration of the Volcano Rucu-Pichincha, (Quito ;) by MM. 

 Seb. Wisse et Garcia Moreno, during August, 1845, (abridged from 

 the Comptes Rendus, 1846 ; Quart. Jour. Geol. Soc, No. 10.) 

 Pichincha is situated eleven miles in a straight line W.N.W. from Quito: 

 its sides, which are covered with vegetation to the height of 12,116 

 feet, are furrowed by deep ravines. All the part above, called the 

 ' Arena! ,' is covered with sand and pumice, and is inclined at an angle 



of25°to35°. 



The authors having ascended the Arenal to the height of 1542 feet, 

 reached the edge of the crater, which is broken down on the south and 

 on the west, and found the cavity of the volcano to consist of two fun- 

 nel-shaped craters, apparently resulting from two sets of eruptions. 

 They descended into the eastern crater, a depth of 1050 feet, and found 

 it to consist simply of a vast ravine, at the bottom of which was the bed 

 of a torrent, always dry except during rains. 



The western crater is nearly circular, and regularly funnel-shaped : 

 at the bottom is a small plain, through which flow two torrents, which 

 unite near the western opening of the crater. On the western side of 

 •his plain rises a hill or cone of eruption, whose height is about 260 

 feet above the mean level of the bottom of the crater, and its diameter 

 about 1476 feet. This hill is embraced by the two torrents, so as to 

 form a kind of peninsula during heavy rains. It is far from perfectly 

 conical at present, being covered with irregular heaps of stones, and 

 fissured in all directions, proving the violence of the convulsions it has 

 been subjected to in recent times. The volcanic vents, whether active 

 or extinct, are all situated in this cone of eruption ; not the slightest 

 trace of one being found elsewhere. They are arranged in nearly cir- 

 cular groups of dTflerent dimensions, some of them attaining a diameter 

 of 82 feet. There are in all nine of these groups, six in activity and 

 three extinct, all situated in those parts of the 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 four inches in breadth, from which issue vapors ; 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 reacned ; 

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

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

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

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



Secord Series, Vol. IV, No. 12—Nov., 1847. 53 



