Ce. XXIX.] CALDERA OF PALMA. 633 



fragment. Each imbedded stone is either angular, or, if globular, con- 

 sists of scoriae more or less spongy, and evidently not owing its shape 

 to attrition. It would be impossible to account for the absence of 

 water-worn pebbles if the coarse breccia in question had been spread 

 by aqueous agency over a horizontal area coextensive with the Caldera 

 and the volcanic rocks which surround it. The only cause known to 

 us capable of dispersing such heavy fragments, some of them 3, 4, or 

 or 6 feet in diameter, without blunting their edges, is the power of 

 steam, unless indeed we could suppose that ice had cooperated with 

 water in motion ; and the interference of ice cannot be suspected in 

 this latitude (28° 40'), especially as I looked in vain for signs of 

 glacial action here and in the other mouutainous regions of the 

 Canary Islands. 



The lower formation of the Caldera is, as before stated, equally of 

 igneous origin. It differs in its prevailing color from the upper, ex- 

 hibiting a tea-green and in parts a light yellow tint, instead of the 

 usual brown, lead-colored, or reddish hues of basalt and its associated 

 scoriae. Beds of a light greenish tuff are common, together with tra- 

 chytic and greenstone rocks, the whole so reticulated by dikes, some 

 vertical, others oblique, others tortuous, that we found it impossible 

 to determine the general dip of the beds, although at the head of the 

 great gorge or Barranco they certainly dip .outwards, or to the south, 

 as stated by Yon Buch. But in following the section down the same 

 ravine, where the mountain called Alejanado (d, figs. pp. 628 and 631) 

 is cut through, and where the rocks of the lower formation are very 

 crystalline, we found what is not alluded to by the Prussian geologist, 

 that the beds exposed to view in cliffs 1500 feet high have an anticli- 

 nal arrangement, exhibiting first a southerly and then a northerly dip 

 at angles varying from 20 to 40 degrees (see section, fig. 699, at k). 

 Hence we may presume that the older strata must have undergone 

 great movements before the upper formation was superimposed. No 

 organic remains having been discovered in the older series, we can- 

 not positively decide whether it was of subaerial or submarine origin. 

 We can only affirm that it has been produced by successive eruptions, 

 chiefly of felspathic lavas and tuffs. Many beds which probably con- 

 sisted at first of soft tuffs have been much hardened by the contact of 

 dikes and apparently much altered by other plutonic influence, so that 

 they have acquired a semicrystaliine and almost metamorphic char- 

 acter. 



The existence of so great a mass of volcanic rocks of ancient date 

 on the exact site of an equally vast accumulation of comparatively 

 modern lavas and scories is peculiarly worthy of notice as a general 

 phenomenon observed in very different parts of the globe. It proves 

 that, not withstanding the fact in the past history of volcanoes that 

 one region after another has. been for ages and has then ceased to be 

 the chief theatre of igneous action, still the activity of subterranean 

 heat may often be persistent for more than one geological period in 



