632 CALDERA OF PALMA. [Ch. XXIX. 



gravel and conglomerate lower down at I and i, are newer than all the 

 volcanic rocks seen in this section. 



The upper volcanic formation, to he described in the sequel, is 

 traversed by numerous dikes, which could not be expressed on this 

 small scale. The vertical lines in the lower formation represent a few 

 of the perpendicular dikes which abound there. Countless others, 

 inclined and tortuous, are found penetrating the same rocks. The five 

 outliers of somewhat pyramidal shape, at the bottom of the Caldera 

 (on each side of m), agree in structure and composition with the upper 

 formation, and may have subsided into their present position, if the 

 Caldera was caused by engulfment, or may have slid down in the 

 form of land-slips, if the cavity be attributed chiefly to aqueous 

 erosion. 



In the description above given of the section" (fig.. 699), the cliffs 

 which wall in the Caldera are spoken of as consisting of two forma- 

 tions. Of these the uppermost alone gives rise to vertical precipices, 

 from the base of which the lower descends in steep slopes, which, 

 although they have the external aspect of taluses, are not in fact made 

 up of broken materials, or of ruins detached from the higher rocks, but 

 consist of rocks in place. Both formations are of volcanic origin, but 

 they differ in composition and structure. In the upper, the beds con- 

 sist of agglomerate, scoriae, lapilli, and lava, chiefly basaltic, the whole 

 dipping outwards, as if from the axis of the original cone, at angles 

 varying from 10 to 28 degrees. The solid lavas do not constitute 

 more than a fourth of the entire mass, and are divided into beds of 

 very variable thickness, some scoriaceous and vesicular, others more 

 compact, and even in some cases rudely columnar. All these more 

 stony masses are seen to thin out and come to an end wherever they 

 can be traced horizontally for a distance of half or a quarter of a mile, 

 and usually sooner. Coarse breccias or agglomerates predominate in 

 the lower part, as if the commencement of the second series of rocks 

 marked an era of violent gaseous explosions. Single beds of this 

 aggregate of angular stones and scoriae attain a thickness of from 200 

 to 300 feet. They are united together by a paste of volcanic dust or 

 spongiform scoriae. 



At one point on the right side of the great Barranco, near its exit 

 from the Caldera, we observed in the boundary precipice a lofty 

 column of amorphous and scoriaceous rock in which the red or rust- 

 colored scoriae are as twisted and ropy as any to be seen on the slopes 

 of Vesuvius ; seeming to imply that there was here an ancient vent or 

 channel of discharge subsequently buried under the products of newer 

 eruptions. Countless dikes, more or less vertical, consisting chiefly of 

 basaltic lava, traverse the walls of the Caldera, some of them terminat- 

 ing upwards, but a great number reaching the very crest of the ridge, 

 and therefore having been posterior in origin to the whole precipice. 



We could not discover in any one of the fallen masses of agglom- 

 erate which strewed the base of the cliffs a single pebble or water-worn 



