Mr. J. Smith on the Geology of Madeira. 171 



the other volcanic products, and in dykes which intersect all the 

 igneous rocks. It is occasionally amorphous, but more often rudely 

 columnar ; it also, though rarely, presents regular columns ; and 

 it is sometimes schistose, possessing planes of cleavage as well as of 

 regular stratification. 



The scoriaceous basalt is rough and porous, resembling the slag 

 of a foundry. Where the bed is thin it is scoriaceous throughout, 

 but where it is of a certain thickness only the upper and lower sur- 

 faces exhibit this character. Two caverns of considerable magni- 

 tude occur immediately to the west of the principal landing-place 

 at Funchal, and there are others in the island. 



The vesicular lava or basalt presents through its whole mass a 

 porous texture. The large vesicles have been flattened by the gra- 

 vity of the lava, and elongated in the direction in which the coulee 

 flowed. Where they are numerous and minute, they permit the 

 rock to be easily hewn ; and this variety is called cantaria rija, or 

 the hard building stone. 



The lapilli, sand, ashes and volcanic bombs, appear to have been 

 projected simultaneously, as the bombs were evidently half imbedded 

 in the finer materials by the force of their fall, the laminae beneath 

 them being bent upwards, and in some instances to a greater height 

 on one side than the other, indicating the direction in which the 

 bombs fell. 



The pumiceous lapilli are white or light yellow, and rarely ex- 

 ceed in size a pigeon's egg. Beds of pumice, varying in thickness 

 from a few inches to several feet, occur either on the surface or in- 

 terstratified with the basalt and tufa ; and they often contain por- 

 tions of heavier volcanic products, as cinders or scoriae, dispersed 

 without regard to gravitation, proving, Mr. Smith says, that these 

 various materials could not have been deposited under the sea, be- 

 cause in water they would instantly have separated according to 

 their respective weights. The scoriae or cinders also form extensive 

 beds. They are generally reddish, and vary in size according to 

 the distance from the orifice of eruption. 



The ashes, both dark and light-coloured, are incoherent, except 

 where they are mixed with earthy matter, or apparently fell on a 

 heated cone of eruption ; and in these cases they form a scoriaceous 

 mass. Tufas and conglomerates compose a large proportion of the 

 volcanic rocks of Madeira, and are considered to owe their consist- 

 ency to water. Fragments of vegetables are not uncommon in them, 

 but Mr. Smith is not aware that they contain any other organic 

 remains. Many of these beds have been converted into vegetable 

 soils, and it is interesting, the author says, to observe the roots of 

 plants still in the attitude in which they grew ; and to witness traces 

 of the very same phsenomena which are now taking place at the sur- 

 face, in strata which have been buried for so many ages under solid 

 rocks. The remains of plants are chiefly found in the vegetable 

 soils, but their roots occasionally occur in the hard rock, and the 

 cracks or fissures are in many cases filled with closely-matted masses 

 of what was once roots and fibres, but now consist of carbonate of 



