62 Mr Scrope's Observations on 



cavity does, under certain circumstances, offer the same phe- 

 nomenon ; but the converse of this is by no means true ; and 

 to produce this effect it is enough that the soil should be 

 of loose, light, and porous materials, so as to contain nume- 

 rous small cavities or interstices. Not only the bottom of the 

 crater, but the external slopes of every volcanic cone, and 

 every flat spot, however distant from any volcanic orifice, which 

 has a moderate coating of fragmentary scoriae or volcanic ash- 

 es, returns this sound on percussion. Not only the sides of 

 Vesuvius, but the whole surface of the Campagna di Roma, 

 and the Terra di Lavoro, must be suspended over a yawning 

 gulf, if this phenomenon is a sufficient proof of such a posi- 

 tion. 



But even all made ground returns a more or less sonorous 

 reverberation when struck sharply, and the causes which pro- 

 duce this effect are well known to natural philosophers. This 

 sound would therefore be produced in the case of Malpais, as 

 naturally by a superficial coating of volcanic ashes as by any 

 vast cavity, did such indeed exist, underneath. 



2. The frequent formation of cracks and fissures across the 

 plain, far from proving the existence of such a subterranean 

 gulf, is a circumstance which accompanies the cooling and 

 consolidation of every bed of lava ; and as these crevices are 

 formed only in the lava (contracting as it congeals) it is to 

 be expected that local subsidences must often take place in 

 the coating of volcanic ashes or black clay, immediately above 

 the clefts. The washing of this clay by rains into the fis- 

 sures of the lava bed beneath, is another probable cause of 

 such subsidences ; much more probable, I should conceive, than 

 the supposition of a natural arched cavity or bubble, four 

 square miles in extent. 



3. A further confirmation of the existence of a bed of lava, 

 beneath the plain of Malpais, is obtained from the disappear- 

 ance of two rivers beneath its surface ; for this accident ne- 

 cessarily results in the instance of all lava currents which 

 have occupied the bed of a river; in consequence of the 

 numerous fissures, with which they are penetrated, but par- 

 ticularly of the bed of loose and cellular scoriae on which they 

 invariably rest. This phenomenon occurs in repeated in- 



