METAMORPHIC ROCKS. 247 



has traversed the banter sandstone and greywacke slate, and 

 has spread itself out, in the form of a cup ; sometimes form- 

 ing groups of columns, and sometimes divided into thin 

 laminse. This, however, is not the case with granite, syenite, 

 porphyritic quartz, serpentine, and the whole series of 

 unstratified rocks, to which, by a predilection for mytho- 

 logical nomenclature, the term plutonic has been applied. 

 With the exception of occasional veins, all these rocks have 

 been forced up in a semi-fluid or pasty condition through 

 large fissures and wide gorges, instead of gushing in a 

 liquid stream from small orifices ; and they are never found 

 in narrow streams like lava, but in extensive masses. ( 26 ) 

 Some groups of dolorites and trachytes shew traces of a 

 degree of fluidity resembling that of basalt; others, form- 

 ing vast craterless domes, appear to have been elevated 

 in a simply softened state , others again, like the trachytes 

 of the Andes, in which I have often remarked a striking 

 analogy to the greenstone and syenitic porphyries (argentife- 

 rous and then without quartz), are found in beds like 

 granite and quartzose porphyry. 



Direct experiments ( 261 j on the alterations which the 

 texture and chemical constitution of rocks undergo from 

 the action o heat have shewn, that volcanic masses (diorite, 

 augitic porphyry, basalt, and the lava of Etna) give different 

 products according to the pressures under which they are 

 melted and the rate at which they are cooled; if 

 the cooling has been rapid, they form a black glass, homo- 

 geneous in the fracture ; if slow, a stony mass, of granular 

 or crystalline structure; and in this latter case crystals 

 are formed in cavities, and even in the body of the 

 mass in which they are imbedded. The same materials also 



