ORIGIN OP ERUPTIVE AND PRIMARY ROCKS. 343 



bined with an evident distension of their crystalline constituents 

 in a direction parallel with the course of the lava stream. Accord- 

 ing to Spallanzani and Dolomieu this phenomenon is of great im- 

 portance, since it was doubtless occasioned by the moving forward 

 and the distension of the half-fluid lava, an explanation amply 

 confirmed by the elongation in the direction of the stream of the 

 cavities filled with gas which are contained in the lava. In the 

 Leucit-lava of Borghetto the crystals of Leucit in spite of their 

 tesseral form are even drawn out in the direction of the stream.* 

 These instances of parallel structure among the trachytic and bas- 

 altic rocks have been specially dwelt upon, because of the analogy 

 they present to gneiss and other schistose rocks of the primitive 

 gneiss formation. 



Porphyry, Greenstone, and Melapliyr. — It has been already 

 mentioned, that the trachytic and basaltic rocks first make 

 their appearance about the commencement of the tertiary 

 period. Instances of such rocks occur however even earlier 

 in the trias formation, in passing backward through which 

 we find that their character gradually changes. Porphyries result 

 on the one hand, and melaphyrs, or commonly called traps, result 

 on the other. The rocks usually comprehended under the nam® 

 melaphyr are, according to Cotta, of a very indefinite character, 

 and resolvable partly into basalt, partly into greenstones, and 

 partly into porphy rites (porphyries free from quartz). On this 

 account it would appear advisable to classify most of the eruptive 

 rocks, which have been protruded during the Silurian, Carbonifer- 

 ous and- Permian periods into two great divisions, viz : porphyries 

 and greenstones. With regard to the igneous origin of these, I 

 cannot do better than quote the argument of Naumann.f " We 

 " have seen, that if the rocks of the lava family (as no one doubts) 

 " must be regarded as pyrogenous formations, then the rocks of 

 " the basalt and trachyte families have a similar origin. If now 

 " the melaphyrs (or traps) are compared with the basalts, and 

 " the felsitic porphyries with the trachytic porphyries, an aston- 

 " ishing similarity will be observed to exist between them ; a 

 " similarity which renders it often quite impossible to distinguish 

 " the one from the other, when hand specimens of them merely 

 " are examined. According to Bergmann and Delesse, we may 

 " recognize the same mineralogical constituents in melaphyr as in 



* Lehrbuch, I, 468. 

 t Lehrbuch, I, 121, 



