220 Professor Hoffmann on (he Geological Investigations 



their dark, often blackish-green colour ; they never contain 

 entirely separated grains of quartz, but consist essentially of 

 augite or pyroxene, which likewise causes their dark colour. 

 The two kinds of porphyry are always distinctly separated 

 from each other ; Buch termed the first red or quartzose por- 

 phyry, and the second black or augite porphyry {melaphyre of 

 Brongniart). 



The mutual relations of these porphyries are extremel}'^ 

 singular. It was found that the red porphyry must be of 

 much more ancient origin than the black ; for, where the two 

 are in contact, not only, as already stated, are they unusually 

 sharply and distinctly separated from each other, but the one 

 is likewise always penetrated and broken up by the other. 

 But the different influence exercised by these two porphyries 

 on the position of the surrounding rocks was of still greater 

 importance. The red porphyry is in this respect indifferent ; it 

 occurs in conformable union with the distinctly stratified red 

 sandstone which is always associated with it, and on which 

 repose conformably the limestones of theliigher chains of the 

 ^Ips. Such, however, is by no means the case with the me- 

 laphyres. Everywhere in their vicinity, phenomena of dis- 

 turbance and breaking up present themselves ; it is true they 

 are often surrounded by a peculiar conglomerate, which is 

 without the stratification exhibited by the red sandstone, and 

 in which the fragments are confusedly mingled, and lie to- 

 gether without any prevailing faces of stratification. It cannot 

 be doubted that this conglomerate has been formed by friction 

 and division of parts at the place where it is now found, and 

 not of rolled fragments deposited from large masses of water. 

 There is not a single rock in the neighbourhood which occurs 

 in conformable relations with the black porphyries ; they 

 have sometimes raised the rocks to the surface, which are now 

 partly surrounded and supported by them in large fragments, 

 and sometimes they cut through and tear asunder all the 

 younger rocks, and have altered in the most remarkable man- 

 ner their original positions. Wherever in that district a sud- 

 denly appearing alternation presents itself at the surface ; 

 wherever precipitous, sharply serrated, inaccessible rocky cones 

 arise, there the black porphyry is not far distant, and occurs 



