m EXCURSIONS IN MADEIRA 



different races of terrestial molluscae which inhabited the soil. 

 For the soil must then have been formed of that calcareous tufa 

 which we now find beneath the sandstone, and on which the 

 living helices are still scattered in prodigious quantities, whilst 

 they are not to be met with on the sandy soil e. Perhaps the 

 system of winds for this part of the Atlantic has been modified 

 in the course of ages \ and the south may have prevailed more 

 frequently formerly, when the outlines, if not the number, of the 

 continents and islands in this sea were different. 



I took a boat and went to the small island of Baxo, one mile 

 and a half in its length, (which bears about N.N.E. and S.S.W.) 

 and half a mile in its greatest breadth. It is only half a mile 

 distant from the south-west end of Porto Santo (entirely composed 

 of chffs of tufa with six dikes), from which it has been evidently 

 separated. Half this intervening space of water is occupied by 

 a bank, and there are only five fathoms in the deepest part of the 

 narrow channel, which is also obstructed by rocks. To get at all 

 the strata in succession, I was obhged to climb up an almost 

 perpendicular height of about 220 feet, on my hands and knees ; 

 not daring once to look belaud me, and frequently shoved and 

 dragged up by my guide, who conducted me down by a com- 



s M. Bremontier, who has examined and studied sandy deposits of this nature 

 (dunes) very thoroughly, estimates their progress at sixty feet in the year, in some 

 parts, and seventy-two in others. In 2000 years they will arrive at, and cover Bor- 

 deaux, (as they have, already, several villages of the Gulf of Gascony) according to 

 his calculation ; and, from their present extent, rather more than 4000 years must 

 have elapsed since their formation commenced. Cuvier, Discours sur la Theorie de 

 la Terre, p. 76. 



^ The evidences of a change of chmate having taken place in the northern temperate 

 zone, are numerous : (See Bovvdich's Elements of Conchology, Introduction, p. viii.) 

 and the influence of differences of climate on prevailing winds has been admirably 

 investigated by de Humboldt. De Vlnjluence de la Declinaison du soleil sur Ic 

 Commencement des Pluies Equatoriales. Annales de Chimie, 1821, p. 179. 



