8 GEOLOGICAL MEMOIRS. 



Trachyte presents all the characters of an igneous rock ; it has 

 "been melted, or at least softened, and rendered plastic by heat. When 

 it is filled with quartz, we see its distinctive characters gradually 

 disappear, and it passes insensibly into porphyry: there is every 

 reason to believe that heat, then, plays a less and less active part in 

 its formation. 



§ 17. Dolerite. — Dolerite, if we give this name to the lavas with 

 anorthose and anhydrous ejected by volcanos, offers in a still more 

 decided degree the characters of an igneous rock. 



It is formed of anorthose and augite ; sometimes it contains 

 peridote, mica, and amphigene (or leucite). These minerals are 

 disseminated in a grey or blackish anhydrous paste, more or less 

 magnetic, and cellular. When this paste predominates and is very 

 cellular, the dolerite passes into a scoria. 



The anorthose has a vitreous lustre ; it is white, translucid, and 

 laminated. It never assumes a greenish colour and a fatty lustre, 

 as in basalt. Consequently there is the same difference between the 

 anorthose of dolerite and that of basalt, as between the orthose of 

 trachyte and that of granite. 



The augite is black or blackish-green, in crystals terminated at 

 both ends. It has also a glassy lustre, as is likewise the case with the 

 peridote and amphigene. 



The author further on observes : There can be no doubt respecting 

 the mode of formation of the rock to which I here give the name of 

 dolerite ; for it is actually ejected by several existing volcanos. Thus 

 the lava of Etna contains labrador and augite; that of Vesuvius 

 contains amphigene, augite, and peridote. Dolerite has therefore been 

 reduced to the liquid state by the action of heat. To this therefore 

 the word " lava," by which several kinds of volcanic rocks are de- 

 scribed, should be specially applied. 



§ 18. Trachyte and dolerite offer us two types of igneous rocks, 

 the origin of which is well known, since we can see them formed in 

 existing volcanos. They contain no appreciable quantity of water ; 

 for that which they may have contained has escaped by means of 

 fumarolles at the moment of their solidification. This water more- 

 over has spread itself through the cavities and fissures of the eruptive 

 rock itself, and to a certain distance in the neighbouring rocks. It 

 has produced chalcedonj^, opal, hyalite, quartz, carbonates, zeolites, 

 and generally all the minerals which fill the cavities of the amygda- 

 loidal rocks. Thus the effects of heat may be complicated by those 

 of water, even when the eruptive rocks are igneous and anhydrous. 



II. Pseudo-igneous Bocks. 



§ 19. Pseudo-igneous rocks have been reduced to a state of fusion 

 partly igneous, partly aqueous. Water, heat, and perhaps also 

 pressure, have all helped to make them plastic. They are always in 

 the state of hydrates. They have often also a cellular structure ; but 

 their minerals have only a very slight vitreous lustre. They separate 

 into prisms, or rather spheroids. They are generally associated with 

 igneous rocks, and they are chiefly met with in volcanic districts. 



