i o St. J ago. 



PAUT I. 



matter beneath the lava, and especially that forming the 

 crystalline spicula between the interstices of the scoriae, 

 although heated in an atmosphere probably composed 

 chiefly of steam, could not have been subjected to the 

 effects of a passing stream ; and hence it is, perhaps, 

 that they have retained their carbonic acid, under a 

 small amount of pressure. 



The fragments of scoriae, embedded in the crystalline 

 calcareous basis, are of a jet black colour, with a glossy 

 fracture like pitchstone. Their surfaces, however, are 

 coated with a layer of a reddish-orange, translucent 

 substance, which can easily be scratched with a knife ; 

 hence they appear as if overlaid by a thin layer of rosin. 

 Some of the smaller fragments are partially changed 

 throughout into this substance : a change which appears 

 quite different from ordinary decomposition. At the 

 Galapagos Archipelago (as will be described in a future 

 chapter), great beds are formed of volcanic ashes and 

 particles of scoriae, which have undergone a closely 

 similar chancre. 



The extent and horizontality of the calcareous stratum. 

 — The upper line of snrface of the calcareous stratum, 

 which is so conspicuous from being quite white and so 

 nearly horizontal, ranges for miles along the coast, at 

 the height of about sixty feet above the sea. The sheet 

 of basalt, by which it is capped, is on an average eighty 

 feet in thickness. Westward of Porto Praya beyond 

 Red Hill, the white stratum with the superincumbent 

 basalt is covered up by more recent streams. North- 

 ward of Signal Post Hill, I could follow it with my 

 eve, trending" awav for several miles along- the sea cliffs. 

 The distance thus observed is about seven miles ; but 

 I cannot doubt from its regularity that it extends much 

 farther. In some ravines at right angles to the coast, 

 it is seen gently dipping towards the sea. probably with 



