220 N'ew Ohervations on Volcanoes and their Lavai, 



with jasper; and Mr. Kirvvan after him, and also father De 

 la Torre, were persuaded that Vesuvius and vEtna are not 

 products of fire, their bases beiiig, as they say, marine stones. 



I must fur. her remark, that if we refuse to regard as pro- 

 ducts of the fire ail the volcanic substances that our furnaces 

 vitrify in a higher degree than the volcaixoes do, there are 

 very few which can be considered as marine, since our fur- 

 naces reduce them to a much more perfect vitrification. 



The basaltic porphyries mentioned by M. Huniboldt may 

 rather be the source of all the porphyries he mentions. It 

 is even doubtful if the ChimboraQO is really a volcano. It 

 is almost probable, he says, that it is of a volcanic nature. 

 If it is only a probability in his idea, it is not astonishing 

 that the substances which constitute it appeared to him to 

 be porphyries, or any other natural rock. 



I collected upon the sea shore at Portici several fragments 

 of rolled lava, which, if we were determined to sec nothing 

 else than porphyries, might very easily be taken for as many 

 varieties of this rock. 



" The porphyries in the neighbourhood of Riobamba and 

 Tunguragua (s;iys M. Humboldt) are 2080 toises thick." 



According to the measurements made at Peru by the 

 French academicians (wliich I shall have occasion to men- 

 tion again) the height of Tunguragua is idSS toises above the 

 level of the sea ; and that of the soil of Quito, situated in the 

 same vallev with Riobamba, is 146iJ There remain, therefore, 

 1161 toises as the elevation of the Tunguragua above the 

 valley. Its form is a rcuul.ir conp, which is the form of all 

 (Volcanoes, and this cone, which is prolonged outwards far 

 below the valley, is composed of volcanic substances onlv; 

 for, as 1 have already, remarked, every volcano, w^hatever 

 be its heiffht, is an accumulation of subtanccs forced out 

 8uccessivelv by one or more moutiis. In this state of things 

 wc could scarcely find room at Tunguragua, and its envi- 

 rons, lor a thickness of porphyry of 20S0 toises. 



Three years ago M. Hergen, professor of mineralogy in 

 the Museum of Madrid, who is mentioned by JVl. Hum- 

 boldt, sent me, among other things, three specimens of 

 substances froju Tunguragua, nanicdj according to eti- 

 quette. 



