•21. EXCURSIONS IN MADEIRA 



fusedly in this conglomerate, beneath, and between the porous 

 basalt at Praia Formosa. To the eastward of Funchal, this con- 

 glomerate matter increases in proportion, and is insinuated between 

 the more compact masses of basalt in vast patches, as if it had 

 flowed down, and been deposited at the same time with it. The 

 olivine of the compact basalt near the coast, and in the immediate 

 environs of Funchal, is generally granular ; it was not evident in 

 any state in the scoriaceous basalt. I observed several pieces of 

 basalt in the walls about Madeira, containing mammillated car- 

 bonate of Ume ; the mammillae about the size of small shot, gene- 

 rally separate, and always in a cell, sometimes large enough to 

 contain several ; so that they would appear to be infiltrations ; 

 but I very rarely found it in the compact basalt near the sea, and 

 presume, these stones were broken from the fragments washed 

 down by the torrents from the interior. The fer oxydule prin- 

 cipally characterizes the red tufa (which indurates by exposure to 

 the air, and forms a building stone), though it sometimes affords 

 long flat prisms of common horneblende, and acts more powerfully 

 on the needle than the yellow, which sometimes contains small 

 glistening particles of feldspatli : both give out innumerable bub- 

 bles of air when put into water ; but I shall have occasion to 

 describe them more particularly, when speaking of the best soils 

 for the vines of Madeira, and wiU only mention here, that when 

 the red tufa is in immediate contact with the porous basalt above 

 it, (as in the ravine descending to the beach) it is formed into 

 small pentagonal prisms, about 2 inches in length, and if in diai- 

 meter ; in this case it is of a reddish brown, looks like a baked 

 clay, and its specific gravity is increased to 2.06. Some of the 

 pumice fragments (evidently not ejected until the scoriaceous 

 basalt had flowed and deposited itself), imbedded in the yellow 

 tufa of the ravine by which I descended to the beach, contained 

 minute crystals of horneblende ; its colour was yellowish, its struc- 

 ture more frequently porous than cellular, rarely fibrous, (therefore 



