AND PORTO SANTO. 81 



as a militia-man, to accompany, and direct me, in my rambles 

 through the island. 



The lowest visible deposit in the island of Porto Santo, is a 

 calcareous tufa, of a greenish-grey colour, wliich extends, in the 

 north-eastern parts of the island, to a height of 1600 feet, and is 

 ribbed throughout with numerous vertical dikes, of a reddish- 

 brown basalt. The middle of the island affords a plain, or rather 

 a shallow basin of sandstone, on a level with the sea on the south 

 side, where it covers the beach with a siliceous sand, which, as 

 we walk to the eastward, gradually becomes mingled with the 

 black ferruginous sand resulting from the decomposition of the 

 tufa. Following this plain in its greatest length, that is, from the 

 beach on the south side, to the Fonte Araya, which is immediately 

 above the beach on the north side, (a distance of 2j miles, and 

 forming the breadth of the middle part of the island) we find 

 ourselves on a sloping cliff, 418 feet above the sea. We may 

 descend this cliff with ease for 134 feet, where the sandstone 

 terminates, being superposed on the tufa, which is here 284 feet 

 deep, (that is, from its junction with the sandstone, to the surface 

 of the waters wliich hide it) and is still intersected by basaltic 

 dikes, which have evidently descended through it, from the highest 

 peaks of the interior of the island. 



The lowest bed of tliis sandstone (which may be best examined 

 in the excavations near the southern beach, being hidden by sand 

 and debris at Araya) is hard and solid, and is used as a building- 

 stone. It is of a reddish buff-colour, of a slaty structure, with 

 indurated veins, eflPervesces pretty vigorously, and presents small 

 black spots, apparently ferruginous. This gradually passes into a 

 looser sandstone (best seen at Araya), of a lighter buff colour 

 within, acquiring a blackened scoriaceous appearance on its outer 

 surface, and of a less stratified appearance, whilst the still looser 

 sandstone above it, presents horizontal bands in the vicinity of 



M 



