68 EXCURSIONS IN MADEIRA 



As I returned, I could not but remark the beautiful hedges of 

 rosemary and naturalized pelargonia, even at a height of 2000 feet 

 above the sea. 



Having no more facts to offer on the Geology of Madeira, I will 

 venture to submit some concluding remarks. The probability, that 

 Madeira and Porto Santo, from their vicinity to the Canaries, 

 belong to the same system of formation, leads us to suspect, even 

 before we have examined them, that they cannot have been 

 created by a marine volcano ™. One thing is clear at first sight, 

 viz., that the masses of basalt have not pre-existed as rock of a 

 different nature, and were afterwards heated in situ, and pene- 

 trated by vapours : every appearance indicates, that these masses 

 have been elevated as fluid, and streamed from the mouth of a 

 crater. It next occurs to us, that had the island of Madeira been 

 entirely created by a marine volcano, its base, if not its bulk, 

 would, probably, (arguing from analogy) be composed of pumice 

 and cinders ; both of which are found in comparatively small 

 quantities, and alternating with basalt and tufa. The discovery 

 of the vast bed of transition limestone below the basalt, and con- 

 tinuing to a depth of 700 feet, until its approach to the level of 

 the sea allows us to trace it no further, confirms our conclusion, 

 and seems to demonstrate, that Madeira pre-existed as a mass 

 of transition, or probably of primitive and transition rocks, after- 

 wards rent by a marine volcano, which covered and elevated 

 the island by successive streams and ejections of basalt and tufa. " 



to Porta S. Jorge, 12 G. miles, according to Almeida, and 12J according to Johnston. 

 I make the circumference by Johnston's map, about 96 G. miles. 



™ Although M. Broussonet's assertion, that the island of Gomera contains a mass 

 of granite and mica-slate, remains unconfirmed, yet M. Escolar has since found a 

 block of primitive sienite in Fortaventura, and Baron Von Buch has found another 

 primitive rock in Palma. — HiimbolcWs Relation Historique, Supplement, p. 640. 



" Had the basalt and tufa of Madeira been formed, or deposited, beneath the sur- 



