ORIGIN OF ERUPTIVE AND PRIMARY ROCKS. 345 



midst of granites and syenites, in which they form veins, so that 

 they are generally newer than these latter rocks. A few of them 

 are decidedly older than the coal period, and several have been 

 formed simultaneously with the Bundtsandstein of the Triassic, and 

 even in the Jurassic and the chalk formations, but the height of 

 their development falls in the Carboniferous and the first part of 

 the Permian period, in the German Rothliegendes. In the latter 

 formation the porphyries have played a very important part, fur- 

 nishing the material for many of its sedimentary rocks, and dis- 

 locating its strata considerably by their intrusion. In the carbo- 

 niferous system porphyries break through and materially disturb 

 the strata, forming veins or dykes, and inserting themselves hori- 

 zontally as layers. While, as we have already mentioned, the 

 basalts and trachytes exert a powerful chemical action on the 

 rocks with which they come in contact, the influence of the 

 prophyries seems to have been almost exclusively of a mechan- 

 ical nature. It seems as if the porphyritic material on its arrival 

 in the upper parts of the earth's crust did not possess such a high 

 temperature or such a great degree of fluidity as the basalts. On 

 the other hand, the rocks broken through by the porphyries, show 

 evideuce of the enormous violence to which they have been sub- 

 jected, huge pieces having been broken off, surrounded by the por- 

 phyritic material, carried off by it and crushed and pulverized in 

 its further progress. In this way have been formed the numer- 

 ous breccias which occur in veins and masses of porphyry, 

 where they adjoin the side-r.ocks. Sometimes the mechanical 

 action has been so violent as to produce even a more finely di- 

 vided material, which in the form of a sandstone-like or clay-like 

 substance constitutes the selvages of many porphyritic veins. By 

 far the most conclusive proofs however of the enormous forces 

 which were at work during the eruption of the porphyry, are 

 to be found in the dislocations which whole systems of strata 

 have undergone. The neighbouring beds have been raised 

 up, folded and fractured, while friction-grooves, and surfaces worn 

 smooth by the sliding of one mass upon another occur at the junc- 

 tion of the erupted rock with the neighbouring strata. These effects 

 furnish almost as conclusive evidence of the igneous origin of the 

 porphyries as the chemical changes on the adjacent rocks do, as to 

 the igneous origin of basalt. 



With regard to the greenstones, they seem to have made their 

 appearance in very great profusion during the Silurian and Devon- 

 Can. Nat. 23 Vol. VIII. 



