348 ORIGIN OP ERUPTIVE AND PRIMARY ROCKS, 



been formed similar to the conglomerates and tuffs of the volca- 

 nic formations, and probably in a similar manner. Moreover just 

 as in these formations we find among the massive rocks above 

 enumerated many instances of undoubtedly igneous rocks pos- 

 sessing a slaty structure. Many feldsitic porphyries possess a streak- 

 ed texture caused sometimes by bands of varying colour, and 

 oftener by the arrangement of the quartz grains or crystals in 

 parallel layers, or the presence of thin lamina? of quartz in 

 the paste.* The instances of a similar modification of structure 

 among the greenstones are very numerous, and they are even 

 more important as showing more clearly the cause of this struc- 

 ture among igneous rocks. The dio'rites usually occur in the 

 form of veins, irregular masses (typhonische Stocke,) and layers. 

 The veins sometimes exhibit the following remarkable phenomena. 

 In the middle they consist of granular diorite, and at the sides of 

 slaty diorite or hornblende slate, a gradual transition being gene- 

 rally observable from the granular to the stratified rock. Some- 

 what similar instances of this nature have already been referred 

 to in the paragraph concerning the basaltic rocks. The cause of 

 these phenomena may most reasonably be sought for in the circum- 

 stances attending the cooling of the rock, and they are most 

 likely the same as those which occasioned a similar structure 

 among the porphyries. The fluid rock of the diorite vein was 

 probably in motion in the centre, while the parts adjoining the 

 side walls were solidified. The current in the centre would have 

 a distending and arranging action at the junction of the fluid with 

 the solidified parts, and an elongation and parallel grouping of the 

 minerals there being formed would be the consequence. Not 

 only has this slaty texture been observed in connection with veins, 

 but it has also been remarked, that the more irregular masses 

 of diorite assume a slaty structure towards their junction with 

 the other rocks, the stratification being, as in the case of the 

 veins, parallel with the line of such junction. Naumann adduces 

 numerous instances of this sort ;f and from a former paper of 

 mine it will be observed, that they often occur in Norway.J 

 Among the melaphyrs or traps the same circumstance is often 

 remarked. In the melaphyr region south of the Hundsriick this 

 rock, when it occurs in veins, often possesses a degree of 

 parallel structure sufficient to cause it to separate into flags, which 



* Lehrbuch, I, 616. 



t Lehrbuch, II, 403. 



X Canadian Naturalist, VII, 115. 



