n6 Galapagos Archipelago. pabtl 



could in few cases have been distinguished; and, con- 

 sequently, this wide undulatory tract might have, (as 

 probably many tracts have,) been erroneously considered 

 as formed of one great deluge of lava, instead of by a 

 multitude of small streams, erupted from many small 

 orifices. 



In several parts of this tract, and especially at the 

 base of the small craters, there are circular pits, with 

 perpendicular sides, from twenty to forty feet deep. At 

 the foot of one small crater, there were three of these 

 pits. They have probably been formed, by the falling 

 in of the roofs of small caverns. 1 In other parts, there 

 are mammiform hillocks, which resemble great bubbles 

 of lava, with their summits fissured by irregular cracks, 

 which appeared, upon entering them, to be very deep; 

 lava has not flowed from these hillocks. There are, 

 also, other very regular, mammiform hillocks, composed 

 of stratified lava, and surmounted by circular, steep- 

 sided hollows, which, I suppose have been formed by a 

 body of gas, first, arching the strata into one of the 

 bubble-like hillocks, and then, blowing off its summit. 

 These several kinds of hillocks and pits, as well as the 

 numerous, small, scoriaceous craters, all show that this 

 tract has been penetrated, almost like a sieve, by the 

 passage of heated vapours. The more regular hillocks 

 could only have been heaved up. whilst the lava was in 

 a softened state. 2 



1 M. Elie de Beaumont has described (' Mem. pour servir,' &c, 

 torn. iv. p. 113) many 'petits cirques d'eboulement ' on Etna, of some 

 of which the origin is historically known. 



2 Sir G-. Mackenzie (' Travels in Iceland,' pp. 389 to 392) has de- 

 scribed a plain of lava at the foot of Hecla, everywhere heaved up into 

 great bubbles or blisters. Sir George states that this cavernous lava 

 composes the uppermost stratum : and the same fact is affirmed by 

 Von Buch ('Descript. des Isles Canaries,' p. 159), with respect to the 

 basaltic stream near Bialejo, in Teneriffe. It appears singular that 

 it should be the upper streams that are chiefly cavernous, for one 

 sees no reason why che upper and lower should not have been equally 



