m EXCURSIONS IN MADEIRA 



these dikes to the sea, immediately north of Pico Juliana, 

 (apparently the highest in the island, yet not much exceeding that 

 of Castello) I found a deposit of native alum ; and almost on a 

 level with the sea, another of a bright orange ochre"", accompanied 

 by a ferruginous spring. The tufa would seem to be strongly 

 impregnated with muriate of soda, for all the streams which issue 

 from it, at whatever height, are very brackish, whilst the water 

 from the sandstone is always pure, even when the sources are 

 much nearer the sea than those of the tufa". I saw a large piece 

 of fibrous gypsum at the Governor s, and it was said to be found 

 in abundance in the north-eastern rocks, or islets, of Pescada and 

 Lorenzo, and in a third, about two miles from the shore. I should 

 have made a point of visiting these rocks ; the rock of Fonte 

 (about a mile from the northern shore), the island of Farol (about 

 half a mile from Fachoda, the W.S.W. point of Porto Santo), 

 which appeared to me to be covered with scoriaceous basalt, over- 

 laying the tufa, and dipping rapidly to the W.S.W., and the island 

 of da Serra about half a mile from Dos Frades, the S.S.E. point of 

 Porto Santo ; — but to do this, which would not have occupied two 

 days more, I must have given up the opportunity of returning 

 with the Genoese, and hired a boat at my own expense, to carry 

 me back to Madeira, which was out of the question. I was pre- 

 vented from examining the Desertas (which have evidently been 

 divided by the strong south-west current that sets against them), 

 by similar considerations. 



There are no traces of primitive or secondary formations in 



■" This ochre was evidently formed from the decomposition of the basalt, for on 

 breaking a lump, a nucleus of basalt was found within it. It is a richer pigment than 

 the ochre imported at Madeira from Teneriffe, to paint the doors and wainscots of the 

 houses, in imitation of mahogany. 



" In this respect, it approaches Baron de Humboldt's argile muriatifere of Araya, 

 near Cumana, also found nearly 13,000 feet above the sea, on the Cordillieres of New 

 Grenada. Relation Historique, 1. 2, c. 5. 



