Neoi Olservaiions on Volcanoes and their Lava. 221 



quette, turimgagua. One of these specimens is of an almost 

 compact lava; the otiicr is of a spongy lava; and the tliird 

 >s a pumice stone. The compact lava contains several small 

 whitish crystalline laniinrs, which strongly resemble those 

 of the lavas of ^ina, and give to this lava an appear-tnce of 

 porphyry, which may have induced M. Hmr.boldl to regard 

 it as porphycy. This volcano is towards the cast, opposite 

 to ChimboracOj and some leagues north from Riobamba. 



M. Bouguer, one of the academicians sent to Peru in 

 order to measure the degrees of the meridian, gives a n)ore 

 precise idea of (he substances which compose the volcanoes 

 of that countrv. Upon his return from this long and labo- 

 rious voyage, he gave an account of it to the Academy of 

 Sciences on the 14!h of November l 744, and we find in his 

 discourse the follovvlng observations* : 



'* We have an opportunity at Peru of seeing the interior 

 of the earth to a great depth, because it is every where cut 

 into deep ravines. Many of them have been discovered 

 200 toises broad, and 60 or SO deep ; some are even twice 

 as large. It is only necessary to descend into them in order 

 to see all the qualities of the different beds ; no fossil what- 

 ever is discovered in tJieu]. We see plenty of that black 

 kind of sand which is attracted by the magnet ; and we are 

 able to ascertain, in general, that the layers we observe there, 

 and the shades of which are very distinct, far from beincr 

 the effect of difierer.t alluvial deposits, are rather caused by 

 the expatision of the substances vomited forth by volcanoes; 

 almost every thing seen in these ravines being the ^vork of 

 fird. Some of these mountains, even to a great depth, arc 

 formed of nothing else than scoriae, pumice-stones, and 

 fragments of burnt stones of all sizes; and sometimes the 

 wlmle is concealed under a layer of common earth, which 

 produces herbs and even trees, I have seeii beds of burnt 

 stones reduced into very small morsels, almost as thick as 

 the height of five or six men, chiefly at the bottom of the 

 Cotopaxi, which has becou)e a perfect truncated cone, and 

 of which the suunnit lias been carried off. The bottom of 



♦ Moinoirs of the Acadeniy of 'icicnces fur 1711 ; Kh cdiiion, p. "70. 



this 



