AND PORTO SANTO. 9 



entrance of the aqueduct, I found above the Umestone (but without 

 witnessing the contact), a basalt composed of a dark grey feldspath, 

 full of ochry red and yellow streaks and spots, mixed with shining 

 scales of oxide of iron, and having a granite-hke texture and 

 appearance ; it seemed to descend in sheets, to the south. The 

 glass disclosed innumerable small grains of green earth, and it 

 passed into decomposing masses so thickly speckled with it, as to 

 assume the appearance of a porphyritic sandstone. Examining 

 the specimens I had separated from the blocks of basalt on the 

 opposite side, with a glass, I found they presented the same 

 spotted structure, with the addition of small crystals of hornblende. 

 The upper part of a decomposing mass of this basalt, dipping 

 towards the south, contained laminae of talc passing into steatite, 

 and terminated eastward in a deposit of a deep-red ferruginous 

 earth. As I ascended to a wall, close behind the statue at the 

 entrance of the aqueduct, I found several blocks of basalt, similar 

 to those on the western height, and creeping through a large hole 

 in this wall to examine a hillock of the decomposing basalt (the 

 exterior surface of which yielded to the finger, and was profusely 

 speckled with green-earth), I picked up several loose pieces of 

 amygdaloid : the small oval cavities were generally filled with a 

 dull yellowish earth, and the cellular mass was of the same speckled 

 basalt. In one of the small hillocks there was a portion of con- 

 glomerate, composed of earthy and crystallized lime, minute scales 

 of talc, and blunt fragments of red feldspath ; but the mass was 

 so small, so abruptly discontinued, and so nearly parallel with the 

 basaltic rock by the side of it, that I can say nothing of their 

 relative age. The dip of the limestone seemed to be to the 

 eastward, for its depth diminished in that direction ; and it dis- 

 api^eared on that side, at about 400 feet below the height to which 

 ip reached on the western side. I had not time to follow it south- 



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