58 Mr Scrope's Observations on 



currents were produced by the apertures, marked by the other 

 cones, they do not show themselves ; but the plain from which 

 these hills rise exhibits a great intumescence, or convexity of 

 surface. Its superficial soil consists of horizontal layers of a 

 black clay, in which augite crystals are thickly disseminated. 

 The same clay, but enveloping concentric balls of basalt^ com- 

 poses the numerous little hollow conical protuberances, (or 

 bubbles,) with which the surface of the plain is sprinkled. 



Now, on comparing these appearances with those which re- 

 sult from ordinary volcanic eruptions, little other difference is 

 perceivable than that the quantity of lava produced, or at 

 least remaining visible, bears but a very small proportion to 

 the violence of the eruptions, and the immense quantity of 

 scoria? thrown out. It seems extraordinary also, that but one 

 out of six cones should have given rise to a lava current. 

 Hence a suspicion arises that a greater quantity of lava was 

 in fact emitted, but that it is concealed by the sprinkling of 

 triturated scoriae or volcanic sand, which these large cones 

 must have thrown out during the latter period of their erup- 

 tions. 



In this manner, the lava currents, produced by the eruption 

 of Vesuvius in October 1822, lie at this moment covered to 

 a depth of from two to ten feet by the finer fragmentary sub- 

 stances ejected by the volcano during the last days of its pa- 

 roxysm. And what renders the analogy still more striking, 

 these ashes, which, from the fineness of their comminution 

 mixed into a retentive paste with the torrents of rain that im- 

 mediately followed the eruption, present the appearance of 

 strata of a black clay, precisely like those described by M. de 

 Humboldt, as forming the surface of the plain of Malpais. 



The convexity of this plain is therefore most naturally ac- 

 counted for by supposing it to form the surface of a great bed 

 of lava, resulting from the union of different currents, which, 

 owino" to the previous flatness of the surface on which they 

 were poured forth, their simultaneous emission in great abun- 

 dance from so many neighbouring orifices, and their very low 

 degree of liquidity,* united into a sort of pool or lake of lava, 



* The very coarse grain of the lava of Jorullo (Dolerite, Humboldt,) 

 warrants this assumption of its extremely imperfect liquidity. 



