THE GEOLOGIST. 



Eeferring to tlie agents whicii, in the interior of the earth, may have 

 aided in rendering rocks plastic, or generally tended to the develop- 

 ment of minerals, he reduces them to four — heat, water, pressure, and 

 molecular action, attributing to the first only a comparatively limited 

 role in the formation of eruptive rocks. 



After describing what he conceives to be the action of these 

 various agents, he thereon founds a classification of all eruptive 

 rocks, according to their mode of origin, into the following divisions : 



I. Igneous eruptive rocks. 



II. Pseudo-igneous eruptive rocks. 



III. Non-igneous eruptive rocks. 



I. Igneous are those which have been reduced to a state of fasion, 

 or at least rendered plastic by heat. They are almost always 

 completely anhydrous, with a cellular structure and a rough feel to 

 the touch, then* constituent minerals having a strongly-marked cha- 

 racteristic vitreous aspect; they constitute the rocks regarded as 

 eminently volcanic. As extreme types of this class he especially 

 refers to, trachyte and dolerite.* 



II. Pseudo-igneous rocks are those that may have been reduced to 

 a state of plasticity partly by igneous and partly by aqueous action. 

 Water, heat, and perhaps pressure may have combined to contribute 

 to this. Rocks of this class have sometimes a cellular, or even 

 scoriaceous structure; but then* constituent minerals have only a 

 slightly vitreous aspect. They are hydrated, often containing 

 zeolites, and dividing into prisms or spheroids, and are generally 

 associated with igneous rocks in volcanic regions. As types of this 

 class he refers to retiaite (pitchstone or phonohte), basalt, and trap.f 

 These two classes represent those rocks usually called volcanic. 



* Trachyte is a rock contaimng orthose and anortliose felspar, ferro-magnesian 

 mica, lioniblende, and also quartz. By anorthose felspar is designated, in a 

 gouonil ^yay, all the felspar species belonging to the sixth crystalline system. 

 Dolevito ia an anorthose and anhydrous lava, composed of anorthose felspar and 

 augite, with sometimes olivine, mica, and leucite. 



t llotinito consists of vitreous orthose felspar, of ferro-magnesian mica, and 

 also (lunvtz rather rarely, and always a large proportion of water, which may rise 

 from 10 to 100 per cent. Pechstein and phonoHte are varieties of this rock. 

 IJaaalt principally consists of anorthose felspar, augite, and ohvine, with some- 

 tinvos prot(ixidc> of iron, carbonates, zeolites ; and accidentally nepheline, hauyne, 

 zn-con, conuuluiu, Ibvniing a hydrated felspathic paste. Basalt has the 



