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On the Relations of Trap-Bocks with the Ores of Copper and 

 Iron, and the SimilarUy of the Schalstein of Dillenburg^ the 

 Blatterstein of the Harz, and the Gabbro of Tuscany. 



I. The Relations of Trap-Rocks with Ores of Copper and Iron. 



Under the denomination of trap-rocks, we comprehend the 

 silicates with isomorphous bases of magnesia, lime, and pro- 

 toxide of iron ; rocks of sombre colours, green and blackish, 

 generally but slightly crystalline, known by the names of 

 greenstone, mandelstein, amphibolite, diorite, amygdaloid, 

 spilite, variolite, euphotide, and serpentine,* forming erup- 

 tive groups distinctly characterised. 



With regard to composition, these rocks, although less sili- 

 cated than the quartzo-felspathic rocks which have preceded 

 and followed them, present us with alumina only in small 

 proportion, either as an accidental substance, or in the eupho- 

 tides, variolites, diorites, or labrador melaphyres, which are 

 found in them in subordinate masses towards the line of 

 contact with the eruptive groups. The trap composition is 

 shewn by the deep colours, green and black, which contrast 

 with all the other rocks that surround them ; the superficial 

 alterations indicate the presence of iron by the ochreous tints, 

 and, when the iron is in small proportion, as in serpentine, the 

 power of resisting decomposition, and the persistence of the 

 mineralcgical characters, still further become a special cha- 

 racteristic, when all the adjoining rocks exhibit the effects of 

 decomposition. 



The trap-rocks are distinguished by their homogeneous 

 characters ; we may traverse for two entire hours the serpen- 

 tine surfaces of Italy and the greenstones of Germany, with- 

 out finding any substances crystallised, and of well defined 

 composition. Diallage, steatite, amphibole, pyroxene, and 

 yenite are the only crystalline minerals whose presence is 



* We do not mean to comprehend, in the series of trap-rocks, all those 

 which bear these denominations; the diorites, for example, belong to the fel- 

 spathic group ; but, in certain countries, particularly in the liarz, these names 

 have been applied to rocks evidently of the trap series. 



