CHAP. VI. 



GEOLOGY. 133 



constituent, affiliates the beds in which it is found to the 

 basaltic or trap rocks. Such are not the products of recent 

 volcanoes any more than of furnaces. The silicioiis sinter 

 formed even by the boiling eruptions of Hecla contains no 

 water. Hydrated silicates seem to belong peculiarly to trap 

 formations. 



The scanty list of other minerals found in Madeira cor- 

 roborates these views. The Zeolites, which in the vicinity of 

 Porta da Cruz abound in the lavas, correspond with the opal 

 of the tufas in the character of being hydrates. Augite and 

 olivine, minerals characteristic of basalt, occur in every rock. 

 Specular and micaceous iron are found near St. Jorge. The 

 specimens of iron pyrites which are occasionally met with 

 may perhaps be of recent formation ; but there are no other 

 minerals of modern aspect, no sulphur, no obsidian or obsi- 

 dianic pumice, no silicious sinter. A small deposit of car- 

 bonaceous matter*, without any marks of organization, is 

 found in the ravine of St. Jorge ; but there are no remains 

 in Madeira of plants, animals, or soil, buried beneath tufa or 

 lava; with the exception of the tertiary bed already described, 

 there is nowhere a trace of organic substance beneath the 

 present " habitats " of organic life. The ramifications of car- 

 bonate of lime which have spread through portions of the 

 tufa, and have been supposed to represent ancient fibres and 

 roots of plants, are for the most part the effect only of capil- 

 lary infiltration, and where tubular structure indicates that 

 they have encrusted vegetable fibres which have since disap- 

 peared, they are not beyond the reach of the roots of recent 



* The following analysis of this lignite is given by Mr. Smith as made 

 by Professor Johnstone : — 



Carbon 60'7 



Hydrogen 5-82 



Oxygen and Nitrogen . . . 33 - 47 



