DELESSE — ORIGIN OF ROCKS. 9 



The types "which I shall describe in the two felspathic series are 

 retinite and basalt. 



§ 20. Retinite. — Retinite contains vitreous orthose, ferro-niag- 

 nesian mica, and also quartz. These minerals are disseminated in a 

 paste which is itself entirely vitreous, and which forms the greatest 

 portion of the rock. 



The paste has a glance at once vitreous and resinous, which is 

 characteristic of retinite (pechstein) ; it is often cellular, and the 

 colour varies from red and brown to green and dark black. It always 

 contains a large proportion of water, sometimes as much as 10 per cent. 



Obsidian may also be considered as a variety of retinite, in which 

 there are only slight traces of water and bituminous or organic 

 matter : in a complete classification of rocks it ought therefore to be 

 placed at the extreme limit of the hydrated igneous rocks. 



The minerals enclosed in retinite are the same as those found in 

 the cavities of trachyte. There is especially much silex in the state 

 of opal, chalcedony, and quartz. Zeolites also occur. 



The absence of carbonates in retinite, which deserves special notice, 

 is doubtless owing to the fact of the retinite being in a vitreous state. 

 Carbonates are generally wanting in those eruptive rocks which are 

 rich in silex, even when they are hydrated. 



Retinite is frequently found in volcanic regions, where it is often 

 associated w T ith trachyte and also basalt. It forms well- characterized 

 veins ; and it is evident that it was completely plastic at the time of 

 its eruption. 



After describing the different positions and modes of occurrence of 

 retinite, the author continues : — 



Phonolite differs much from retinite in its physical properties ; 

 nevertheless these two rocks contain vitreous orthose, and are hy- 

 drated; they are moreover associated with trachyte, so that they 

 represent two different states of hydrated trachyte. 



The study of retinite and all the rocks belonging to the class of 

 vitreous trachytes shows us that these rocks are of mixed origin, and 

 result from the combined action of heat, water, and probably also of 

 pressure. They preserve, it is true, the characters of the igneous 

 rocks, but, instead of being anhydrous, are hydrated ; water, then, 

 has plaved an important part in their formation. 



§ 21. Basalt. — Basalt is then considered; and the author comes 

 to the conclusion that all the properties of basalt distinctly show its 

 mixed origin, and that heat and water have contributed to its forma- 

 tion. It has really undergone an " aqueous fusion." Its temperature 

 was sufficiently high to permit the development of peridotc and augite, 

 but not sufficient, however, to completely disengage the water and 

 volatile substances. We can understand thence why basalt is so 

 often associated with lavas, and why we observe it at great distances 

 from all kinds of volcanos. It should not be inferred, however, from 

 the water it contains tbat its eruption was submarine ; it was 

 formed, on the contrary, in the interior of the earth. 



§ 22. Trap. — At the end of the pseudo-igneous rocks is trap. 

 Closely as it is allied to basalt, I think it differs from it in having 



